init

shared/core/build.gradle

@@ -0,0 +1,2 @@
+// Provider abstraction, Immich client, upload pipeline, configuration.
+// Nothing in this module may import a Minecraft or Fabric type.

shared/core/src/main/java/dev/photosync/core/PhotoSync.java

@@ -0,0 +1,113 @@
+package dev.photosync.core;
+
+import dev.photosync.core.config.ConfigService;
+import dev.photosync.core.config.ConfigStore;
+import dev.photosync.core.config.PhotoSyncConfig;
+import dev.photosync.core.provider.ProviderCatalog;
+import dev.photosync.core.provider.ProviderFactory;
+import dev.photosync.core.provider.ProviderSession;
+import dev.photosync.core.provider.immich.ImmichProviderFactory;
+import dev.photosync.core.thumbnail.ThumbnailLoader;
+import dev.photosync.core.timeline.TimelineBrowser;
+import dev.photosync.core.upload.UploadCoordinator;
+import dev.photosync.core.upload.UploadQueue;
+import lombok.Getter;
+import lombok.extern.slf4j.Slf4j;
+
+import java.nio.file.Path;
+import java.time.Duration;
+import java.util.List;
+
+/**
+ * Everything the mod does that is not Minecraft, assembled.
+ *
+ * <p>This is the seam the platform code sees: a Fabric entrypoint builds one of
+ * these with a config directory and then only ever talks to the services hanging
+ * off it. Nothing below this package knows what a {@code Screen} is, and nothing
+ * here reaches back up.
+ *
+ * <p>The wiring is done in a constructor rather than by a container because
+ * there are seven objects and their order is fixed. The one piece of behaviour
+ * that lives here is the subscription that rebuilds the provider whenever the
+ * player edits their credentials.
+ */
+@Slf4j
+public final class PhotoSync implements AutoCloseable {
+
+    private static final String CONFIG_FILE = "photosync.json";
+    private static final String QUEUE_FILE = "uploads.json";
+
+    @Getter
+    private final ProviderCatalog catalog;
+    @Getter
+    private final ConfigService config;
+    @Getter
+    private final ProviderSession session;
+    @Getter
+    private final UploadQueue queue;
+    @Getter
+    private final UploadCoordinator uploads;
+    @Getter
+    private final TimelineBrowser browser;
+    @Getter
+    private final ThumbnailLoader thumbnails;
+
+    /** The standard set of backends. */
+    public PhotoSync(Path directory) {
+        this(directory, List.of(new ImmichProviderFactory()));
+    }
+
+    /** Takes the backend list explicitly so tests can run against a fake one. */
+    public PhotoSync(Path directory, List<ProviderFactory> providers) {
+        this.catalog = new ProviderCatalog(providers);
+        this.config = new ConfigService(new ConfigStore(directory.resolve(CONFIG_FILE), catalog.preferred()));
+        this.session = new ProviderSession(catalog);
+        // Reads and repairs the on-disk queue, so anything interrupted by the
+        // last quit is already pending again by the time uploads start.
+        this.queue = new UploadQueue(directory.resolve(QUEUE_FILE));
+        this.uploads = new UploadCoordinator(queue, session, () -> config.current().upload());
+        this.browser = new TimelineBrowser(session);
+        this.thumbnails = new ThumbnailLoader(session);
+    }
+
+    /** Connects to the configured backend and starts draining the queue. */
+    public PhotoSync start() {
+        config.onChange(this::applyConnection);
+        applyConnection(config.current());
+        uploads.start();
+        int resumed = queue.activeCount();
+        if (resumed > 0) {
+            log.info("Resuming {} upload(s) left over from the last session", resumed);
+        }
+        return this;
+    }
+
+    /** True while anything is still queued, uploading or waiting to retry. */
+    public boolean isBusy() {
+        return queue.hasActiveWork();
+    }
+
+    /**
+     * Drains the queue, giving up after {@code timeout}.
+     *
+     * <p>Answers whether it finished, which is what the quit dialog reports back
+     * to the player. Whatever is left is already on disk and will be picked up
+     * the next time the game starts.
+     */
+    public boolean drain(Duration timeout) {
+        return uploads.shutdown(timeout);
+    }
+
+    private void applyConnection(PhotoSyncConfig current) {
+        session.configure(current.provider(), current.connection());
+        uploads.wake();
+    }
+
+    @Override
+    public void close() {
+        thumbnails.close();
+        browser.close();
+        uploads.close();
+        session.close();
+    }
+}

shared/core/src/main/java/dev/photosync/core/capture/CaptureOrigin.java

@@ -0,0 +1,7 @@
+package dev.photosync.core.capture;
+
+/** Whether the player pressed the key, or the auto-capture timer fired. */
+public enum CaptureOrigin {
+    MANUAL,
+    AUTOMATIC
+}

shared/core/src/main/java/dev/photosync/core/capture/CapturedScreenshot.java

@@ -0,0 +1,24 @@
+package dev.photosync.core.capture;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.time.Instant;
+
+/**
+ * A screenshot that has just landed in the screenshots folder.
+ *
+ * <p>Produced by the platform layer -- which is the only part that knows how
+ * Minecraft writes the file -- and consumed by the upload queue.
+ */
+public record CapturedScreenshot(Path file, String fileName, Instant capturedAt, CaptureOrigin origin, long sizeBytes) {
+
+    public static CapturedScreenshot of(Path file, CaptureOrigin origin) throws IOException {
+        return new CapturedScreenshot(
+                file.toAbsolutePath(),
+                file.getFileName().toString(),
+                Files.getLastModifiedTime(file).toInstant(),
+                origin,
+                Files.size(file));
+    }
+}

shared/core/src/main/java/dev/photosync/core/config/AutoCaptureSettings.java

@@ -0,0 +1,53 @@
+package dev.photosync.core.config;
+
+import lombok.Builder;
+
+/**
+ * Unattended screenshots on a timer.
+ *
+ * <p>Off by default, and deliberately so: a mod that silently starts writing
+ * files and uploading them the moment it is installed would be a bad neighbour.
+ */
+@Builder(toBuilder = true)
+public record AutoCaptureSettings(
+        boolean enabled,
+        int intervalSeconds,
+        String fileNameSuffix,
+        boolean onlyInWorld,
+        boolean skipWhenScreenOpen) {
+
+    public static final int MIN_INTERVAL_SECONDS = 5;
+    public static final int MAX_INTERVAL_SECONDS = 3600;
+
+    public static AutoCaptureSettings defaults() {
+        return AutoCaptureSettings.builder()
+                .enabled(false)
+                .intervalSeconds(300)
+                .fileNameSuffix("_auto")
+                .onlyInWorld(true)
+                .skipWhenScreenOpen(true)
+                .build();
+    }
+
+    AutoCaptureSettings normalized() {
+        return toBuilder()
+                .intervalSeconds(Math.min(MAX_INTERVAL_SECONDS, Math.max(MIN_INTERVAL_SECONDS, intervalSeconds)))
+                .fileNameSuffix(sanitizeSuffix(fileNameSuffix))
+                .build();
+    }
+
+    /**
+     * The suffix lands in a file name, so anything that would break a path has
+     * to go. An empty result is allowed -- it just means "no suffix".
+     */
+    private static String sanitizeSuffix(String raw) {
+        if (raw == null) {
+            return "";
+        }
+        StringBuilder clean = new StringBuilder(raw.length());
+        raw.chars()
+                .filter(ch -> ch > 0x1F && "\\/:*?\"<>|".indexOf(ch) < 0)
+                .forEach(ch -> clean.append((char) ch));
+        return clean.toString().strip();
+    }
+}

shared/core/src/main/java/dev/photosync/core/config/BrowserSettings.java

@@ -0,0 +1,25 @@
+package dev.photosync.core.config;
+
+import lombok.Builder;
+
+/** Look and memory budget of the timeline browser. */
+@Builder(toBuilder = true)
+public record BrowserSettings(int tileSize, int thumbnailCacheEntries, boolean showVideoBadge) {
+
+    public static BrowserSettings defaults() {
+        return BrowserSettings.builder()
+                .tileSize(96)
+                // Each cached tile is an uploaded GPU texture plus its decoded
+                // bytes, so this is the single biggest memory knob in the mod.
+                .thumbnailCacheEntries(256)
+                .showVideoBadge(true)
+                .build();
+    }
+
+    BrowserSettings normalized() {
+        return toBuilder()
+                .tileSize(Math.min(192, Math.max(48, tileSize)))
+                .thumbnailCacheEntries(Math.min(2048, Math.max(32, thumbnailCacheEntries)))
+                .build();
+    }
+}

shared/core/src/main/java/dev/photosync/core/config/ConfigService.java

@@ -0,0 +1,64 @@
+package dev.photosync.core.config;
+
+import lombok.extern.slf4j.Slf4j;
+
+import java.io.IOException;
+import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.Consumer;
+import java.util.function.UnaryOperator;
+
+/**
+ * Holds the live configuration and tells the rest of the mod when it changes.
+ *
+ * <p>The upload coordinator, the auto-capture timer and the browser all need to
+ * react to a settings edit without the settings screen knowing they exist, so
+ * they subscribe here instead.
+ */
+@Slf4j
+public final class ConfigService {
+
+    private final ConfigStore store;
+    private final AtomicReference<PhotoSyncConfig> current;
+    private final List<Consumer<PhotoSyncConfig>> listeners = new CopyOnWriteArrayList<>();
+
+    public ConfigService(ConfigStore store) {
+        this.store = store;
+        this.current = new AtomicReference<>(store.load());
+    }
+
+    public PhotoSyncConfig current() {
+        return current.get();
+    }
+
+    /**
+     * Subscribes to later changes. Listeners run on whichever thread made the
+     * change -- in practice the client thread, from the settings screen.
+     */
+    public void onChange(Consumer<PhotoSyncConfig> listener) {
+        listeners.add(listener);
+    }
+
+    public void update(UnaryOperator<PhotoSyncConfig> change) {
+        PhotoSyncConfig updated = current.updateAndGet(previous -> change.apply(previous).normalized(previous.provider()));
+        persist(updated);
+        for (Consumer<PhotoSyncConfig> listener : listeners) {
+            try {
+                listener.accept(updated);
+            } catch (RuntimeException e) {
+                log.error("A config listener failed; the change itself was still applied", e);
+            }
+        }
+    }
+
+    private void persist(PhotoSyncConfig config) {
+        try {
+            store.save(config);
+        } catch (IOException e) {
+            // Losing the write is bad but not fatal: the in-memory config is
+            // already updated, so the session behaves as the player asked.
+            log.error("Could not write {}", store.path(), e);
+        }
+    }
+}

shared/core/src/main/java/dev/photosync/core/config/ConfigStore.java

@@ -0,0 +1,73 @@
+package dev.photosync.core.config;
+
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import dev.photosync.core.persistence.JsonFile;
+import dev.photosync.core.provider.ProviderId;
+import lombok.extern.slf4j.Slf4j;
+
+import java.io.IOException;
+import java.nio.file.Path;
+import java.util.Map;
+import java.util.Optional;
+
+/**
+ * Reads and writes {@code photosync.json}.
+ *
+ * <p>Loading merges the stored file <em>onto</em> the serialized defaults rather
+ * than deserializing it directly. A setting added in a later release is therefore
+ * present with its default in an old config file, and a setting removed in a
+ * later release is ignored -- no schema version, no migration step. This is not
+ * a nicety: Gson gives every absent record component {@code null} or {@code 0},
+ * so without the merge a newly added {@code lingerMillis} would arrive as zero
+ * and notifications would vanish for everyone upgrading.
+ */
+@Slf4j
+public final class ConfigStore {
+
+    private final JsonFile file;
+    private final ProviderId fallbackProvider;
+
+    public ConfigStore(Path path, ProviderId fallbackProvider) {
+        this.file = new JsonFile(path);
+        this.fallbackProvider = fallbackProvider;
+    }
+
+    public Path path() {
+        return file.path();
+    }
+
+    /** Never throws: a broken config falls back to defaults rather than blocking start-up. */
+    public PhotoSyncConfig load() {
+        PhotoSyncConfig defaults = PhotoSyncConfig.defaults(fallbackProvider);
+        Optional<JsonElement> stored = file.readTree();
+        if (stored.isEmpty() || !stored.get().isJsonObject()) {
+            return defaults;
+        }
+        try {
+            JsonObject merged = file.gson().toJsonTree(defaults).getAsJsonObject();
+            overlay(merged, stored.get().getAsJsonObject());
+            return file.gson().fromJson(merged, PhotoSyncConfig.class).normalized(fallbackProvider);
+        } catch (RuntimeException e) {
+            log.warn("{} could not be understood; using defaults", file.path(), e);
+            return defaults;
+        }
+    }
+
+    public void save(PhotoSyncConfig config) throws IOException {
+        file.write(file.gson().toJsonTree(config));
+    }
+
+    /** Recursively copies {@code stored} over {@code base}, object by object. */
+    private static void overlay(JsonObject base, JsonObject stored) {
+        for (Map.Entry<String, JsonElement> entry : stored.entrySet()) {
+            JsonElement current = base.get(entry.getKey());
+            JsonElement incoming = entry.getValue();
+            if (current != null && current.isJsonObject() && incoming.isJsonObject()) {
+                overlay(current.getAsJsonObject(), incoming.getAsJsonObject());
+            } else {
+                base.add(entry.getKey(), incoming);
+            }
+        }
+    }
+}

shared/core/src/main/java/dev/photosync/core/config/NotificationKind.java

@@ -0,0 +1,8 @@
+package dev.photosync.core.config;
+
+/** Which of the three things worth a corner message just happened. */
+public enum NotificationKind {
+    CAPTURED,
+    UPLOADED,
+    FAILED
+}

shared/core/src/main/java/dev/photosync/core/config/NotificationSettings.java

@@ -0,0 +1,46 @@
+package dev.photosync.core.config;
+
+import lombok.Builder;
+
+/**
+ * The one-line messages that appear in the bottom-left corner.
+ *
+ * <p>Each event is switchable on its own, because the useful setting for a
+ * player running auto-capture every thirty seconds is different from the one for
+ * a player who screenshots twice an hour.
+ */
+@Builder(toBuilder = true)
+public record NotificationSettings(
+        boolean enabled,
+        boolean onCapture,
+        boolean onUploaded,
+        boolean onFailed,
+        int lingerMillis) {
+
+    public static NotificationSettings defaults() {
+        return NotificationSettings.builder()
+                .enabled(true)
+                .onCapture(true)
+                .onUploaded(true)
+                .onFailed(true)
+                .lingerMillis(3000)
+                .build();
+    }
+
+    NotificationSettings normalized() {
+        return toBuilder()
+                .lingerMillis(Math.min(15_000, Math.max(500, lingerMillis)))
+                .build();
+    }
+
+    public boolean shows(NotificationKind kind) {
+        if (!enabled) {
+            return false;
+        }
+        return switch (kind) {
+            case CAPTURED -> onCapture;
+            case UPLOADED -> onUploaded;
+            case FAILED -> onFailed;
+        };
+    }
+}

shared/core/src/main/java/dev/photosync/core/config/PhotoSyncConfig.java

@@ -0,0 +1,64 @@
+package dev.photosync.core.config;
+
+import dev.photosync.core.provider.AlbumRef;
+import dev.photosync.core.provider.ProviderConnection;
+import dev.photosync.core.provider.ProviderId;
+import lombok.Builder;
+
+/**
+ * The whole of PhotoSync's settings, as one immutable value.
+ *
+ * <p>Screens never mutate this. They build a new one -- {@code
+ * config.toBuilder().albumId(id).build()} -- and hand it to {@link
+ * ConfigService}, which persists it and tells everyone who cares. That makes
+ * "cancel" free and "what changed?" answerable.
+ */
+@Builder(toBuilder = true)
+public record PhotoSyncConfig(
+        ProviderId provider,
+        ProviderConnection connection,
+        String albumId,
+        UploadSettings upload,
+        AutoCaptureSettings autoCapture,
+        NotificationSettings notifications,
+        BrowserSettings browser) {
+
+    public static PhotoSyncConfig defaults(ProviderId provider) {
+        return PhotoSyncConfig.builder()
+                .provider(provider)
+                .connection(ProviderConnection.empty())
+                .albumId("")
+                .upload(UploadSettings.defaults())
+                .autoCapture(AutoCaptureSettings.defaults())
+                .notifications(NotificationSettings.defaults())
+                .browser(BrowserSettings.defaults())
+                .build();
+    }
+
+    /**
+     * Repairs a config that came off disk: fills in anything a hand edit removed
+     * and clamps anything a hand edit made absurd. Called on every load, so the
+     * rest of the mod may assume its values are sane.
+     */
+    public PhotoSyncConfig normalized(ProviderId fallbackProvider) {
+        return PhotoSyncConfig.builder()
+                .provider(provider == null ? fallbackProvider : provider)
+                .connection(connection == null ? ProviderConnection.empty() : connection)
+                .albumId(albumId == null ? "" : albumId.trim())
+                .upload((upload == null ? UploadSettings.defaults() : upload).normalized())
+                .autoCapture((autoCapture == null ? AutoCaptureSettings.defaults() : autoCapture).normalized())
+                .notifications((notifications == null ? NotificationSettings.defaults() : notifications).normalized())
+                .browser((browser == null ? BrowserSettings.defaults() : browser).normalized())
+                .build();
+    }
+
+    /** Where uploads go and what the browser shows. Empty album id means the whole library. */
+    public AlbumRef album() {
+        return AlbumRef.of(albumId);
+    }
+
+    /** True once there is enough here to actually talk to a server. */
+    public boolean isReady() {
+        return connection.isConfigured();
+    }
+}

shared/core/src/main/java/dev/photosync/core/config/UploadSettings.java

@@ -0,0 +1,36 @@
+package dev.photosync.core.config;
+
+import lombok.Builder;
+
+/** How aggressively the queue drains, and what happens when the player quits. */
+@Builder(toBuilder = true)
+public record UploadSettings(
+        boolean uploadOnCapture,
+        int concurrency,
+        int maxAttempts,
+        int retryBackoffSeconds,
+        boolean waitOnQuit,
+        boolean deleteLocalAfterUpload) {
+
+    public static final int MAX_CONCURRENCY = 4;
+
+    public static UploadSettings defaults() {
+        return UploadSettings.builder()
+                .uploadOnCapture(true)
+                .concurrency(2)
+                .maxAttempts(5)
+                // Multiplied by 2^(attempt-1), so 5s grows to 80s by the fifth try.
+                .retryBackoffSeconds(5)
+                .waitOnQuit(true)
+                .deleteLocalAfterUpload(false)
+                .build();
+    }
+
+    UploadSettings normalized() {
+        return toBuilder()
+                .concurrency(Math.min(MAX_CONCURRENCY, Math.max(1, concurrency)))
+                .maxAttempts(Math.min(20, Math.max(1, maxAttempts)))
+                .retryBackoffSeconds(Math.min(300, Math.max(1, retryBackoffSeconds)))
+                .build();
+    }
+}

shared/core/src/main/java/dev/photosync/core/package-info.java

@@ -0,0 +1,12 @@
+/**
+ * Everything PhotoSync does that is not about Minecraft.
+ *
+ * <p>Nothing under {@code dev.photosync.core} may import a Minecraft, Fabric or
+ * LWJGL type. That rule is what lets this module be compiled once, to Java 17
+ * bytecode, and folded unchanged into all nine platform jars. It is also what
+ * makes the upload pipeline testable without a game running.
+ *
+ * <p>The dependency arrow only ever points inwards: platform code depends on
+ * {@code :shared:ui}, which depends on {@code :shared:mc-api} and this module.
+ */
+package dev.photosync.core;

shared/core/src/main/java/dev/photosync/core/persistence/Adapters.java

@@ -0,0 +1,74 @@
+package dev.photosync.core.persistence;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonToken;
+import com.google.gson.stream.JsonWriter;
+import dev.photosync.core.provider.ProviderId;
+
+import java.io.IOException;
+import java.time.Instant;
+import java.time.format.DateTimeParseException;
+
+/**
+ * The handful of value types PhotoSync persists that Gson would otherwise write
+ * as nested objects.
+ *
+ * <p>Everything else -- records, enums, primitives -- Gson already handles, and
+ * on purpose: the config and queue models are built out of types the library
+ * understands so that this class stays this short. Note the Gson version we
+ * compile against (2.10, the oldest Minecraft ships in the supported range) is
+ * also the first that can construct records, which is what makes that possible.
+ */
+final class Adapters {
+
+    private Adapters() {
+    }
+
+    static Gson newGson() {
+        return new GsonBuilder()
+                .registerTypeAdapter(ProviderId.class, new ProviderIdAdapter().nullSafe())
+                .registerTypeAdapter(Instant.class, new InstantAdapter().nullSafe())
+                .setPrettyPrinting()
+                .disableHtmlEscaping()
+                .create();
+    }
+
+    /** Writes {@code "immich"} rather than {@code {"key":"immich"}}. */
+    private static final class ProviderIdAdapter extends TypeAdapter<ProviderId> {
+
+        @Override
+        public void write(JsonWriter out, ProviderId value) throws IOException {
+            out.value(value.key());
+        }
+
+        @Override
+        public ProviderId read(JsonReader in) throws IOException {
+            String key = in.nextString();
+            return key.isBlank() ? null : new ProviderId(key);
+        }
+    }
+
+    /** ISO-8601, so the queue file stays readable when someone goes looking. */
+    private static final class InstantAdapter extends TypeAdapter<Instant> {
+
+        @Override
+        public void write(JsonWriter out, Instant value) throws IOException {
+            out.value(value.toString());
+        }
+
+        @Override
+        public Instant read(JsonReader in) throws IOException {
+            if (in.peek() == JsonToken.NUMBER) {
+                return Instant.ofEpochMilli(in.nextLong());
+            }
+            try {
+                return Instant.parse(in.nextString());
+            } catch (DateTimeParseException e) {
+                return null;
+            }
+        }
+    }
+}

shared/core/src/main/java/dev/photosync/core/persistence/JsonFile.java

@@ -0,0 +1,121 @@
+package dev.photosync.core.persistence;
+
+import com.google.gson.Gson;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonParser;
+import lombok.extern.slf4j.Slf4j;
+
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.AtomicMoveNotSupportedException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.StandardCopyOption;
+import java.nio.file.attribute.PosixFilePermission;
+import java.util.Optional;
+import java.util.Set;
+
+/**
+ * One JSON document on disk, written so that it cannot be found half-written.
+ *
+ * <p>PhotoSync keeps two of these -- the settings and the upload queue -- and
+ * both have the same requirement: the game can be killed at any moment, and what
+ * survives has to be either the old file or the new one. Writing in place would
+ * make a hard shutdown during a save look identical to "the mod lost my API key"
+ * or "the mod lost my queue", so every write goes to a scratch file and is moved
+ * over the target.
+ */
+@Slf4j
+public final class JsonFile {
+
+    private final Path path;
+    private final Gson gson;
+
+    public JsonFile(Path path) {
+        this.path = path;
+        this.gson = Adapters.newGson();
+    }
+
+    public Path path() {
+        return path;
+    }
+
+    /** Configured with PhotoSync's persistence conventions -- see {@link Adapters}. */
+    public Gson gson() {
+        return gson;
+    }
+
+    public boolean exists() {
+        return Files.isRegularFile(path);
+    }
+
+    /**
+     * Parses the file, or returns empty when it is missing, unreadable or not
+     * valid JSON. Callers fall back to a default rather than failing start-up:
+     * losing settings is annoying, refusing to launch is worse.
+     */
+    public Optional<JsonElement> readTree() {
+        if (!exists()) {
+            return Optional.empty();
+        }
+        try {
+            String text = Files.readString(path, StandardCharsets.UTF_8);
+            JsonElement parsed = JsonParser.parseString(text);
+            return parsed.isJsonNull() ? Optional.empty() : Optional.of(parsed);
+        } catch (IOException | RuntimeException e) {
+            log.warn("Could not read {}; falling back to defaults", path, e);
+            quarantine();
+            return Optional.empty();
+        }
+    }
+
+    public void write(JsonElement tree) throws IOException {
+        Path directory = path.toAbsolutePath().getParent();
+        Files.createDirectories(directory);
+
+        Path scratch = Files.createTempFile(directory, path.getFileName() + ".", ".tmp");
+        try {
+            Files.writeString(scratch, gson.toJson(tree), StandardCharsets.UTF_8);
+            restrictToOwner(scratch);
+            try {
+                Files.move(scratch, path, StandardCopyOption.REPLACE_EXISTING, StandardCopyOption.ATOMIC_MOVE);
+            } catch (AtomicMoveNotSupportedException e) {
+                // Network shares and some Windows setups refuse atomic moves. A
+                // plain replace is still better than writing the target in place.
+                Files.move(scratch, path, StandardCopyOption.REPLACE_EXISTING);
+            }
+        } finally {
+            Files.deleteIfExists(scratch);
+        }
+    }
+
+    /**
+     * Keeps a corrupt file around under a {@code .broken} name instead of
+     * silently overwriting it on the next save, so a player who lost their
+     * settings still has something to send us.
+     */
+    private void quarantine() {
+        Path broken = path.resolveSibling(path.getFileName() + ".broken");
+        try {
+            Files.move(path, broken, StandardCopyOption.REPLACE_EXISTING);
+            log.warn("Moved the unreadable file aside as {}", broken);
+        } catch (IOException e) {
+            log.debug("Could not move {} aside", path, e);
+        }
+    }
+
+    /**
+     * The settings file holds an API key. Where the filesystem can express it,
+     * keep the file to its owner; where it cannot, carry on.
+     */
+    private static void restrictToOwner(Path file) {
+        try {
+            if (file.getFileSystem().supportedFileAttributeViews().contains("posix")) {
+                Files.setPosixFilePermissions(file,
+                        Set.of(PosixFilePermission.OWNER_READ, PosixFilePermission.OWNER_WRITE));
+            }
+        } catch (IOException | UnsupportedOperationException e) {
+            log.debug("Could not restrict permissions on {}", file, e);
+        }
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/Album.java

@@ -0,0 +1,15 @@
+package dev.photosync.core.provider;
+
+import java.util.Optional;
+
+/** One album as shown in the album picker. */
+public record Album(String id, String name, int assetCount, Optional<String> coverAssetId) {
+
+    public Album {
+        coverAssetId = coverAssetId == null ? Optional.empty() : coverAssetId;
+    }
+
+    public AlbumRef ref() {
+        return AlbumRef.of(id);
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/AlbumRef.java

@@ -0,0 +1,29 @@
+package dev.photosync.core.provider;
+
+import java.util.Optional;
+
+/**
+ * Which slice of the backend we are talking about: one album, or the whole
+ * library.
+ *
+ * <p>Both browsing and uploading take one of these, and "no album configured"
+ * is a first-class case rather than a null id sprinkled through the code.
+ */
+public record AlbumRef(Optional<String> id) {
+
+    public AlbumRef {
+        id = id == null ? Optional.empty() : id.filter(value -> !value.isBlank());
+    }
+
+    public static AlbumRef library() {
+        return new AlbumRef(Optional.empty());
+    }
+
+    public static AlbumRef of(String albumId) {
+        return new AlbumRef(Optional.ofNullable(albumId));
+    }
+
+    public boolean isLibrary() {
+        return id.isEmpty();
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/AssetKind.java

@@ -0,0 +1,11 @@
+package dev.photosync.core.provider;
+
+/**
+ * PhotoSync never plays video. A video asset is shown as its still preview with
+ * a marker drawn over it, which is why this is a two-value enum and not a media
+ * type hierarchy.
+ */
+public enum AssetKind {
+    IMAGE,
+    VIDEO
+}

shared/core/src/main/java/dev/photosync/core/provider/BucketPage.java

@@ -0,0 +1,11 @@
+package dev.photosync.core.provider;
+
+import java.util.List;
+
+/** The assets of one {@link TimelineBucket}, newest first. */
+public record BucketPage(TimelineBucket bucket, List<RemoteAsset> assets) {
+
+    public BucketPage {
+        assets = assets == null ? List.of() : List.copyOf(assets);
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/PhotoProvider.java

@@ -0,0 +1,70 @@
+package dev.photosync.core.provider;
+
+import java.util.List;
+
+/**
+ * The seam between PhotoSync and whatever is storing the photos.
+ *
+ * <p>Every method is blocking and is only ever called off the render thread, by
+ * the upload workers or the timeline loader. Implementations must be safe to
+ * call from several threads at once.
+ *
+ * <p>The interface is deliberately narrow and read-mostly. PhotoSync uploads,
+ * and it browses; it does not delete, favourite or move remote assets, so no
+ * such capability is exposed here. Keeping it that way is what makes a second
+ * backend a weekend's work instead of a rewrite.
+ */
+public interface PhotoProvider extends AutoCloseable {
+
+    ProviderDescriptor descriptor();
+
+    /**
+     * Verifies that the endpoint answers and the credentials are accepted.
+     *
+     * @throws ProviderException with {@link ProviderException.Kind#AUTHENTICATION}
+     *         if the secret is wrong, or {@link ProviderException.Kind#NETWORK}
+     *         if the endpoint cannot be reached
+     */
+    ProviderIdentity identify() throws ProviderException;
+
+    /** Albums the account can upload into, for the settings picker. */
+    List<Album> albums() throws ProviderException;
+
+    Album createAlbum(String name) throws ProviderException;
+
+    /**
+     * Uploads one file and, when {@link UploadRequest#album()} names an album,
+     * puts it there.
+     *
+     * <p>Must be idempotent with respect to file content: re-uploading bytes the
+     * backend already has has to succeed with
+     * {@link UploadReceipt.Outcome#DUPLICATE} rather than fail or duplicate.
+     * The upload queue leans on this when it retries a job whose response was
+     * lost -- for instance because the game was killed mid-request.
+     */
+    UploadReceipt upload(UploadRequest request, TransferProgress progress) throws ProviderException;
+
+    /**
+     * Every bucket of the timeline with its asset count, newest first.
+     *
+     * <p>One cheap call that gives the browser enough to size its scrollbar for
+     * the entire album without fetching any asset.
+     */
+    List<TimelineBucket> timeline(AlbumRef album) throws ProviderException;
+
+    /** The assets of a single bucket returned by {@link #timeline(AlbumRef)}. */
+    BucketPage page(AlbumRef album, TimelineBucket bucket) throws ProviderException;
+
+    /**
+     * Encoded image bytes for one asset.
+     *
+     * <p>Must be PNG or JPEG. The game decodes these with stb_image, which reads
+     * neither WebP nor AVIF, so a backend that stores those has to transcode or
+     * ask for a different rendition -- the mod cannot recover from it later.
+     */
+    byte[] thumbnail(String assetId, ThumbnailSize size) throws ProviderException;
+
+    /** Releases connections. Never throws, so callers can use try-with-resources cleanly. */
+    @Override
+    void close();
+}

shared/core/src/main/java/dev/photosync/core/provider/ProviderCatalog.java

@@ -0,0 +1,58 @@
+package dev.photosync.core.provider;
+
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+
+/**
+ * The set of backends this build knows about, in the order the settings screen
+ * offers them.
+ *
+ * <p>An instance rather than a static registry: it is created once by
+ * {@code PhotoSync} and handed to whoever needs it, which keeps the wiring
+ * visible and the whole thing constructible in a test.
+ */
+public final class ProviderCatalog {
+
+    private final Map<ProviderId, ProviderFactory> factories;
+    private final ProviderId preferred;
+
+    public ProviderCatalog(List<ProviderFactory> factories) {
+        if (factories.isEmpty()) {
+            throw new IllegalArgumentException("A catalog needs at least one provider");
+        }
+        Map<ProviderId, ProviderFactory> byId = new LinkedHashMap<>();
+        for (ProviderFactory factory : factories) {
+            ProviderFactory clash = byId.put(factory.descriptor().id(), factory);
+            if (clash != null) {
+                throw new IllegalArgumentException("Duplicate provider id: " + factory.descriptor().id());
+            }
+        }
+        this.factories = Map.copyOf(byId);
+        this.preferred = factories.get(0).descriptor().id();
+    }
+
+    /** Used as the default in a fresh config, and as the fallback for an unknown id. */
+    public ProviderId preferred() {
+        return preferred;
+    }
+
+    public List<ProviderDescriptor> descriptors() {
+        return factories.values().stream().map(ProviderFactory::descriptor).toList();
+    }
+
+    public Optional<ProviderDescriptor> describe(ProviderId id) {
+        return Optional.ofNullable(factories.get(id)).map(ProviderFactory::descriptor);
+    }
+
+    /**
+     * Connects using the named provider, falling back to {@link #preferred()}
+     * when the config names one this build does not have -- which happens when a
+     * player downgrades the mod, and is not worth crashing over.
+     */
+    public PhotoProvider connect(ProviderId id, ProviderConnection connection) {
+        ProviderFactory factory = factories.getOrDefault(id, factories.get(preferred));
+        return factory.connect(connection);
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/ProviderConnection.java

@@ -0,0 +1,35 @@
+package dev.photosync.core.provider;
+
+/**
+ * The two things every token-authenticated photo backend needs: where it lives
+ * and how we prove who we are.
+ *
+ * <p>Deliberately not a provider-specific credential bag. Immich calls these a
+ * server URL and an API key; a future backend may call them something else, and
+ * the labels come from {@link ProviderDescriptor} rather than from here.
+ */
+public record ProviderConnection(String endpoint, String secret) {
+
+    public ProviderConnection {
+        endpoint = endpoint == null ? "" : endpoint.trim();
+        secret = secret == null ? "" : secret.trim();
+    }
+
+    public static ProviderConnection empty() {
+        return new ProviderConnection("", "");
+    }
+
+    public boolean isConfigured() {
+        return !endpoint.isEmpty() && !secret.isEmpty();
+    }
+
+    /**
+     * Redacted on purpose. This record ends up in exception messages and debug
+     * logs, and an API key in a pasted log is a real leak.
+     */
+    @Override
+    public String toString() {
+        return "ProviderConnection[endpoint=" + endpoint
+                + ", secret=" + (secret.isEmpty() ? "<unset>" : "<redacted>") + "]";
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/ProviderDescriptor.java

@@ -0,0 +1,45 @@
+package dev.photosync.core.provider;
+
+import java.util.Objects;
+
+/**
+ * What the settings screen needs in order to render a provider's connection
+ * form without knowing which provider it is looking at.
+ *
+ * <p>The point of this type is that adding a second backend means writing a
+ * {@link ProviderFactory} and a language file -- not editing the GUI. The UI
+ * resolves labels by appending fixed suffixes to {@link #translationPrefix()}:
+ *
+ * <ul>
+ *   <li>{@code .name} -- provider name in the picker</li>
+ *   <li>{@code .endpoint} / {@code .endpoint.hint} -- first credential field</li>
+ *   <li>{@code .secret} / {@code .secret.hint} -- second credential field</li>
+ * </ul>
+ */
+public record ProviderDescriptor(ProviderId id, String translationPrefix, boolean supportsAlbums) {
+
+    public ProviderDescriptor {
+        Objects.requireNonNull(id, "id");
+        Objects.requireNonNull(translationPrefix, "translationPrefix");
+    }
+
+    public String nameKey() {
+        return translationPrefix + ".name";
+    }
+
+    public String endpointKey() {
+        return translationPrefix + ".endpoint";
+    }
+
+    public String endpointHintKey() {
+        return translationPrefix + ".endpoint.hint";
+    }
+
+    public String secretKey() {
+        return translationPrefix + ".secret";
+    }
+
+    public String secretHintKey() {
+        return translationPrefix + ".secret.hint";
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/ProviderException.java

@@ -0,0 +1,54 @@
+package dev.photosync.core.provider;
+
+import lombok.Getter;
+
+/**
+ * Anything a backend can fail with, classified well enough for the upload queue
+ * to decide between "retry later" and "stop and tell the player".
+ */
+@Getter
+public class ProviderException extends Exception {
+
+    public enum Kind {
+        /** Connection refused, DNS failure, timeout. Worth retrying. */
+        NETWORK(true),
+        /** Bad or revoked credentials. Retrying will not help. */
+        AUTHENTICATION(false),
+        /** The album or asset is gone. Retrying will not help. */
+        NOT_FOUND(false),
+        /** Server asked us to slow down. Worth retrying, after a longer wait. */
+        RATE_LIMITED(true),
+        /** 5xx. The server may recover. */
+        SERVER(true),
+        /** The response did not look like what the API promised. */
+        PROTOCOL(false),
+        /** The file we were asked to upload is unreadable or gone. */
+        SOURCE_UNREADABLE(false);
+
+        private final boolean retryable;
+
+        Kind(boolean retryable) {
+            this.retryable = retryable;
+        }
+
+        public boolean isRetryable() {
+            return retryable;
+        }
+    }
+
+    private final Kind kind;
+
+    public ProviderException(Kind kind, String message) {
+        super(message);
+        this.kind = kind;
+    }
+
+    public ProviderException(Kind kind, String message, Throwable cause) {
+        super(message, cause);
+        this.kind = kind;
+    }
+
+    public boolean isRetryable() {
+        return kind.isRetryable();
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/ProviderFactory.java

@@ -0,0 +1,17 @@
+package dev.photosync.core.provider;
+
+/**
+ * Creates providers of one kind. Registered in a {@link ProviderCatalog} at
+ * start-up; that registration is the only place a new backend has to be named.
+ */
+public interface ProviderFactory {
+
+    ProviderDescriptor descriptor();
+
+    /**
+     * Builds a provider bound to these credentials. Cheap and non-blocking: no
+     * network call happens until {@link PhotoProvider#identify()} or a real
+     * request, because this runs while the settings screen is being built.
+     */
+    PhotoProvider connect(ProviderConnection connection);
+}

shared/core/src/main/java/dev/photosync/core/provider/ProviderId.java

@@ -0,0 +1,31 @@
+package dev.photosync.core.provider;
+
+import java.util.Locale;
+import java.util.Objects;
+
+/**
+ * Stable key for one backend implementation, for example {@code immich}.
+ *
+ * <p>This value is written to the config file, so it must not change once a
+ * version has shipped.
+ */
+public record ProviderId(String key) implements Comparable<ProviderId> {
+
+    public ProviderId {
+        Objects.requireNonNull(key, "key");
+        key = key.trim().toLowerCase(Locale.ROOT);
+        if (key.isEmpty()) {
+            throw new IllegalArgumentException("A provider id must not be blank");
+        }
+    }
+
+    @Override
+    public int compareTo(ProviderId other) {
+        return key.compareTo(other.key);
+    }
+
+    @Override
+    public String toString() {
+        return key;
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/ProviderIdentity.java

@@ -0,0 +1,8 @@
+package dev.photosync.core.provider;
+
+/**
+ * Proof that the configured credentials work, and something human-readable to
+ * show next to the "Test connection" button.
+ */
+public record ProviderIdentity(String serverVersion, String accountName) {
+}

shared/core/src/main/java/dev/photosync/core/provider/ProviderSession.java

@@ -0,0 +1,88 @@
+package dev.photosync.core.provider;
+
+import lombok.extern.slf4j.Slf4j;
+
+import java.util.Objects;
+import java.util.Optional;
+
+/**
+ * The one live {@link PhotoProvider}, rebuilt whenever the credentials change.
+ *
+ * <p>Uploads and browsing share it, so they also share its connection pool
+ * rather than each opening their own. It knows nothing about the config file --
+ * it is <em>told</em> to reconfigure -- which keeps the provider package free of
+ * a dependency on the config package.
+ */
+@Slf4j
+public final class ProviderSession implements AutoCloseable {
+
+    private final ProviderCatalog catalog;
+    private final Object lock = new Object();
+
+    private ProviderId currentId;
+    private ProviderConnection currentConnection = ProviderConnection.empty();
+    private PhotoProvider provider;
+
+    public ProviderSession(ProviderCatalog catalog) {
+        this.catalog = catalog;
+        this.currentId = catalog.preferred();
+    }
+
+    /** Cheap and idempotent: unchanged credentials leave the provider alone. */
+    public void configure(ProviderId id, ProviderConnection connection) {
+        PhotoProvider discarded = null;
+        synchronized (lock) {
+            if (Objects.equals(currentId, id) && Objects.equals(currentConnection, connection)) {
+                return;
+            }
+            discarded = provider;
+            currentId = id;
+            currentConnection = connection;
+            provider = connection.isConfigured() ? catalog.connect(id, connection) : null;
+        }
+        if (discarded != null) {
+            closeQuietly(discarded);
+        }
+    }
+
+    /** Empty until the player has entered a server and a key. */
+    public Optional<PhotoProvider> provider() {
+        synchronized (lock) {
+            return Optional.ofNullable(provider);
+        }
+    }
+
+    public boolean isConfigured() {
+        synchronized (lock) {
+            return provider != null;
+        }
+    }
+
+    /**
+     * A throwaway provider for the settings screen's "test connection" button,
+     * so the player can check credentials before committing them.
+     */
+    public PhotoProvider probe(ProviderId id, ProviderConnection connection) {
+        return catalog.connect(id, connection);
+    }
+
+    @Override
+    public void close() {
+        PhotoProvider discarded;
+        synchronized (lock) {
+            discarded = provider;
+            provider = null;
+        }
+        if (discarded != null) {
+            closeQuietly(discarded);
+        }
+    }
+
+    private static void closeQuietly(PhotoProvider target) {
+        try {
+            target.close();
+        } catch (RuntimeException e) {
+            log.debug("Closing a provider threw", e);
+        }
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/RemoteAsset.java

@@ -0,0 +1,45 @@
+package dev.photosync.core.provider;
+
+import java.time.Duration;
+import java.time.Instant;
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.util.Optional;
+
+/**
+ * One item in the remote timeline, reduced to what the grid actually draws.
+ *
+ * <p>{@code localCapturedAt} is the wall-clock time where the photo was taken,
+ * not where the player is sitting now. Day grouping uses it, so a screenshot
+ * taken at 00:30 in Tokyo stays on the Tokyo day even when browsed from Berlin
+ * -- which is what every photo app does and what users expect.
+ *
+ * <p>{@code aspectRatio} arrives with the timeline page, before any image data,
+ * so the grid can lay out and reserve space without a single thumbnail request.
+ */
+public record RemoteAsset(
+        String id,
+        AssetKind kind,
+        Instant capturedAt,
+        LocalDateTime localCapturedAt,
+        double aspectRatio,
+        Optional<String> thumbHash,
+        Duration duration) {
+
+    public RemoteAsset {
+        thumbHash = thumbHash == null ? Optional.empty() : thumbHash.filter(hash -> !hash.isBlank());
+        duration = duration == null ? Duration.ZERO : duration;
+        // A zero or negative ratio would divide by zero in the layout pass.
+        if (!(aspectRatio > 0) || Double.isInfinite(aspectRatio)) {
+            aspectRatio = 1.0;
+        }
+    }
+
+    public LocalDate localDay() {
+        return localCapturedAt.toLocalDate();
+    }
+
+    public boolean isVideo() {
+        return kind == AssetKind.VIDEO;
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/ThumbnailSize.java

@@ -0,0 +1,12 @@
+package dev.photosync.core.provider;
+
+/**
+ * Named after what we use the image for rather than after a pixel count, since
+ * every backend has its own idea of what "small" means.
+ */
+public enum ThumbnailSize {
+    /** The tile in the timeline grid. */
+    GRID,
+    /** The single-photo view opened by clicking a tile. */
+    DETAIL
+}

shared/core/src/main/java/dev/photosync/core/provider/TimelineBucket.java

@@ -0,0 +1,24 @@
+package dev.photosync.core.provider;
+
+import java.time.LocalDate;
+
+/**
+ * A month of the remote timeline, and how many assets are in it.
+ *
+ * <p>This is the whole trick behind the virtual list: the backend hands us every
+ * bucket's <em>count</em> in one cheap call, so the scrollbar and the total
+ * height are exact before a single asset has been fetched. Pages are then loaded
+ * only for the months the viewport actually reaches.
+ *
+ * <p>{@code key} is the backend's own opaque token for the bucket and is passed
+ * straight back when requesting its page; {@code month} is the parsed form the
+ * UI sorts and labels with.
+ */
+public record TimelineBucket(String key, LocalDate month, int assetCount) implements Comparable<TimelineBucket> {
+
+    /** Newest first, matching how the timeline is displayed. */
+    @Override
+    public int compareTo(TimelineBucket other) {
+        return other.month.compareTo(month);
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/TransferCancelledException.java

@@ -0,0 +1,21 @@
+package dev.photosync.core.provider;
+
+/**
+ * Thrown out of a {@link TransferProgress} callback to abandon a transfer that
+ * is already on the wire.
+ *
+ * <p>This is how "remove from queue" works on an upload that has started: the
+ * queue flags the job, the next progress callback throws, and the body stream
+ * unwinds. Providers must let it escape rather than wrapping it in a
+ * {@link ProviderException} -- a cancelled upload is not a failure and must not
+ * be retried.
+ *
+ * <p>Unchecked, because it travels through {@link TransferProgress}, which has
+ * no business declaring it.
+ */
+public final class TransferCancelledException extends RuntimeException {
+
+    public TransferCancelledException(String jobId) {
+        super("Transfer " + jobId + " was cancelled", null, false, false);
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/TransferProgress.java

@@ -0,0 +1,20 @@
+package dev.photosync.core.provider;
+
+/**
+ * Byte-level progress callback for an in-flight upload.
+ *
+ * <p>Called from the upload worker thread, possibly very often. Implementations
+ * must not block and must not touch the render thread directly.
+ *
+ * <p>Throwing {@link TransferCancelledException} from here aborts the transfer.
+ */
+@FunctionalInterface
+public interface TransferProgress {
+
+    void onProgress(long bytesTransferred, long totalBytes);
+
+    static TransferProgress ignored() {
+        return (transferred, total) -> {
+        };
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/UploadReceipt.java

@@ -0,0 +1,16 @@
+package dev.photosync.core.provider;
+
+/** What the backend did with an {@link UploadRequest}. */
+public record UploadReceipt(String assetId, Outcome outcome) {
+
+    public enum Outcome {
+        /** A new asset was stored. */
+        CREATED,
+        /**
+         * The backend already had a byte-identical asset and kept the original.
+         * This is a success, not an error: it is exactly what we want to happen
+         * when a retry follows an upload whose response we never saw.
+         */
+        DUPLICATE
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/UploadRequest.java

@@ -0,0 +1,23 @@
+package dev.photosync.core.provider;
+
+import java.nio.file.Path;
+import java.time.Instant;
+
+/**
+ * One file to push to the backend.
+ *
+ * <p>Carries no checksum and no device identifier: those are backend-specific
+ * and are derived inside the provider that needs them.
+ */
+public record UploadRequest(
+        Path file,
+        String fileName,
+        Instant capturedAt,
+        Instant modifiedAt,
+        AlbumRef album) {
+
+    public UploadRequest {
+        album = album == null ? AlbumRef.library() : album;
+        modifiedAt = modifiedAt == null ? capturedAt : modifiedAt;
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/immich/ImmichApi.java

@@ -0,0 +1,226 @@
+package dev.photosync.core.provider.immich;
+
+import com.google.gson.Gson;
+import com.google.gson.JsonSyntaxException;
+import dev.photosync.core.provider.ProviderConnection;
+import dev.photosync.core.provider.ProviderException;
+import dev.photosync.core.provider.TransferCancelledException;
+import dev.photosync.core.provider.TransferProgress;
+
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.lang.reflect.Type;
+import java.net.URI;
+import java.net.URLEncoder;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+import java.nio.charset.StandardCharsets;
+import java.time.Duration;
+import java.util.Map;
+import java.util.Optional;
+import java.util.StringJoiner;
+
+/**
+ * Everything HTTP about talking to Immich: URL shape, authentication, timeouts,
+ * and turning a status code into a {@link ProviderException} the upload queue
+ * can act on.
+ *
+ * <p>Keeping this apart from {@link ImmichProvider} means the provider reads as
+ * a list of API calls and their mapping to PhotoSync's model, with no transport
+ * noise in between.
+ */
+final class ImmichApi {
+
+    private static final Duration CONNECT_TIMEOUT = Duration.ofSeconds(10);
+    private static final Duration REQUEST_TIMEOUT = Duration.ofSeconds(30);
+    /** Uploads get their own budget: a screenshot over a slow uplink is not a hung server. */
+    private static final Duration UPLOAD_TIMEOUT = Duration.ofMinutes(10);
+
+    private final HttpClient http;
+    private final URI root;
+    private final String apiKey;
+    private final Gson gson = new Gson();
+
+    ImmichApi(ProviderConnection connection) {
+        this.root = apiRoot(connection.endpoint());
+        this.apiKey = connection.secret();
+        this.http = HttpClient.newBuilder()
+                .connectTimeout(CONNECT_TIMEOUT)
+                // HTTP/1.1 on purpose. Immich is nearly always behind a
+                // self-hosted reverse proxy, and multipart uploads over an
+                // upgraded HTTP/2 connection are the first thing to break when
+                // that proxy is misconfigured.
+                .version(HttpClient.Version.HTTP_1_1)
+                .followRedirects(HttpClient.Redirect.NORMAL)
+                .build();
+    }
+
+    URI root() {
+        return root;
+    }
+
+    /**
+     * Turns whatever the player typed into the API root.
+     *
+     * <p>People paste {@code immich.example.com}, {@code https://immich.example.com/},
+     * and {@code https://example.com/immich/api} in roughly equal measure, and
+     * being told "connection failed" because of a missing scheme is a miserable
+     * first five minutes with a mod.
+     */
+    static URI apiRoot(String endpoint) {
+        String value = endpoint.trim();
+        if (!value.matches("(?i)^[a-z][a-z0-9+.-]*://.*")) {
+            value = "https://" + value;
+        }
+        while (value.endsWith("/")) {
+            value = value.substring(0, value.length() - 1);
+        }
+        if (!value.endsWith("/api")) {
+            value = value + "/api";
+        }
+        return URI.create(value);
+    }
+
+    <T> T get(String path, Map<String, String> query, Type type) throws ProviderException {
+        return decode(send(request(path, query, REQUEST_TIMEOUT).GET().build()), type);
+    }
+
+    byte[] getBytes(String path, Map<String, String> query) throws ProviderException {
+        return send(request(path, query, REQUEST_TIMEOUT).GET().build());
+    }
+
+    <T> T post(String path, Object body, Type type) throws ProviderException {
+        HttpRequest request = request(path, Map.of(), REQUEST_TIMEOUT)
+                .header("Content-Type", "application/json")
+                .POST(HttpRequest.BodyPublishers.ofString(gson.toJson(body), StandardCharsets.UTF_8))
+                .build();
+        return decode(send(request), type);
+    }
+
+    <T> T put(String path, Object body, Type type) throws ProviderException {
+        HttpRequest request = request(path, Map.of(), REQUEST_TIMEOUT)
+                .header("Content-Type", "application/json")
+                .PUT(HttpRequest.BodyPublishers.ofString(gson.toJson(body), StandardCharsets.UTF_8))
+                .build();
+        return decode(send(request), type);
+    }
+
+    <T> T upload(String path, MultipartBody body, String checksumBase64, TransferProgress progress, Type type)
+            throws ProviderException {
+        HttpRequest.Builder builder = request(path, Map.of(), UPLOAD_TIMEOUT)
+                .header("Content-Type", body.contentType())
+                // Lets the server recognise bytes it already has and answer
+                // "duplicate" instead of storing a second copy -- which is what
+                // makes retrying an interrupted upload safe.
+                .header("x-immich-checksum", checksumBase64)
+                .POST(body.publisher(progress));
+        return decode(send(builder.build()), type);
+    }
+
+    private HttpRequest.Builder request(String path, Map<String, String> query, Duration timeout) {
+        return HttpRequest.newBuilder(uri(path, query))
+                .header("x-api-key", apiKey)
+                .header("Accept", "application/json")
+                .timeout(timeout);
+    }
+
+    private URI uri(String path, Map<String, String> query) {
+        StringBuilder url = new StringBuilder(root.toString()).append(path);
+        if (!query.isEmpty()) {
+            StringJoiner joiner = new StringJoiner("&");
+            query.forEach((key, value) -> joiner.add(encode(key) + "=" + encode(value)));
+            url.append('?').append(joiner);
+        }
+        return URI.create(url.toString());
+    }
+
+    private static String encode(String value) {
+        return URLEncoder.encode(value, StandardCharsets.UTF_8);
+    }
+
+    private byte[] send(HttpRequest request) throws ProviderException {
+        try {
+            HttpResponse<byte[]> response = http.send(request, HttpResponse.BodyHandlers.ofByteArray());
+            int status = response.statusCode();
+            if (status >= 200 && status < 300) {
+                return response.body();
+            }
+            throw failure(status, response.body());
+        } catch (IOException e) {
+            // A cancelled transfer unwinds through the request body stream, and
+            // the client hands it back wrapped. It is not a network failure and
+            // must not be retried, so dig it out before classifying anything.
+            TransferCancelledException cancelled = findCancellation(e);
+            if (cancelled != null) {
+                throw cancelled;
+            }
+            throw new ProviderException(ProviderException.Kind.NETWORK,
+                    "Could not reach " + root.getHost() + ": " + e.getMessage(), e);
+        } catch (UncheckedIOException e) {
+            throw new ProviderException(ProviderException.Kind.SOURCE_UNREADABLE,
+                    "Could not read the file to upload: " + e.getMessage(), e);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+            throw new ProviderException(ProviderException.Kind.NETWORK, "Interrupted while waiting for Immich", e);
+        }
+    }
+
+    private ProviderException failure(int status, byte[] body) {
+        String detail = describe(status, body);
+        ProviderException.Kind kind = switch (status) {
+            case 401, 403 -> ProviderException.Kind.AUTHENTICATION;
+            case 404 -> ProviderException.Kind.NOT_FOUND;
+            case 408, 429 -> ProviderException.Kind.RATE_LIMITED;
+            default -> status >= 500 ? ProviderException.Kind.SERVER : ProviderException.Kind.PROTOCOL;
+        };
+        return new ProviderException(kind, detail);
+    }
+
+    /** Immich answers errors as JSON; fall back to the raw text when it does not. */
+    private String describe(int status, byte[] body) {
+        String text = new String(body, StandardCharsets.UTF_8).trim();
+        Optional<String> message = Optional.empty();
+        if (text.startsWith("{")) {
+            try {
+                ImmichDtos.ApiError error = gson.fromJson(text, ImmichDtos.ApiError.class);
+                message = Optional.ofNullable(error).map(ImmichDtos.ApiError::message);
+            } catch (JsonSyntaxException ignored) {
+                // Fall through to the raw body below.
+            }
+        }
+        String summary = message.filter(value -> !value.isBlank())
+                .orElseGet(() -> text.isEmpty() ? "no details" : abbreviate(text));
+        return "Immich returned HTTP " + status + " (" + summary + ")";
+    }
+
+    private <T> T decode(byte[] body, Type type) throws ProviderException {
+        try {
+            T value = gson.fromJson(new String(body, StandardCharsets.UTF_8), type);
+            if (value == null) {
+                throw new ProviderException(ProviderException.Kind.PROTOCOL, "Immich returned an empty response");
+            }
+            return value;
+        } catch (JsonSyntaxException e) {
+            throw new ProviderException(ProviderException.Kind.PROTOCOL,
+                    "Immich returned something that is not the JSON we expected", e);
+        }
+    }
+
+    private static TransferCancelledException findCancellation(Throwable thrown) {
+        for (Throwable cause = thrown; cause != null; cause = cause.getCause()) {
+            if (cause instanceof TransferCancelledException cancelled) {
+                return cancelled;
+            }
+            if (cause.getCause() == cause) {
+                break;
+            }
+        }
+        return null;
+    }
+
+    private static String abbreviate(String text) {
+        String flat = text.replaceAll("\\s+", " ");
+        return flat.length() <= 160 ? flat : flat.substring(0, 157) + "...";
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/immich/ImmichDtos.java

@@ -0,0 +1,78 @@
+package dev.photosync.core.provider.immich;
+
+import java.util.List;
+
+/**
+ * The slices of the Immich API that PhotoSync actually reads, mirrored as
+ * records for Gson.
+ *
+ * <p>Written against Immich's published OpenAPI document, version 3.1.0. Fields
+ * the mod does not use are simply absent -- Gson ignores unknown JSON members,
+ * so the smaller this file is, the fewer ways a server upgrade can break us.
+ *
+ * <p>Two absences are deliberate rather than incidental:
+ *
+ * <ul>
+ *   <li>{@code duration} is not read. It has changed representation between
+ *       Immich releases (string, then seconds) and the only thing PhotoSync
+ *       would do with it is decorate a badge it already draws from
+ *       {@code isImage}.</li>
+ *   <li>Boxed types throughout, because every one of these arrays is declared
+ *       nullable somewhere in the schema, and an older server that omits one
+ *       should degrade rather than throw.</li>
+ * </ul>
+ */
+final class ImmichDtos {
+
+    private ImmichDtos() {
+    }
+
+    record ServerAbout(String version) {
+    }
+
+    record UserProfile(String name, String email) {
+    }
+
+    record AlbumSummary(String id, String albumName, Integer assetCount, String albumThumbnailAssetId) {
+    }
+
+    record CreateAlbum(String albumName) {
+    }
+
+    /** {@code status} is {@code created} or {@code duplicate}. */
+    record AssetMediaResponse(String id, String status) {
+    }
+
+    record BulkIds(List<String> ids) {
+    }
+
+    record BulkIdResponse(String id, Boolean success, String error) {
+    }
+
+    /** {@code timeBucket} is an opaque token; we hand it straight back to the server. */
+    record TimeBucket(String timeBucket, Integer count) {
+    }
+
+    /**
+     * Immich returns a bucket columnwise: one array per attribute, all the same
+     * length, indexed in parallel. It is an odd shape for a REST API and a very
+     * good one for this use case -- a thousand assets arrive without a thousand
+     * repetitions of every key name.
+     */
+    record BucketAssets(
+            List<String> id,
+            List<String> fileCreatedAt,
+            List<Double> localOffsetHours,
+            List<Double> ratio,
+            List<String> thumbhash,
+            List<Boolean> isImage) {
+
+        int size() {
+            return id == null ? 0 : id.size();
+        }
+    }
+
+    /** The body Immich returns on an error, when it returns one at all. */
+    record ApiError(String message, String error, Integer statusCode) {
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/immich/ImmichProvider.java

@@ -0,0 +1,348 @@
+package dev.photosync.core.provider.immich;
+
+import com.google.gson.reflect.TypeToken;
+import dev.photosync.core.provider.Album;
+import dev.photosync.core.provider.AlbumRef;
+import dev.photosync.core.provider.AssetKind;
+import dev.photosync.core.provider.BucketPage;
+import dev.photosync.core.provider.PhotoProvider;
+import dev.photosync.core.provider.ProviderConnection;
+import dev.photosync.core.provider.ProviderDescriptor;
+import dev.photosync.core.provider.ProviderException;
+import dev.photosync.core.provider.ProviderIdentity;
+import dev.photosync.core.provider.RemoteAsset;
+import dev.photosync.core.provider.ThumbnailSize;
+import dev.photosync.core.provider.TimelineBucket;
+import dev.photosync.core.provider.TransferProgress;
+import dev.photosync.core.provider.UploadReceipt;
+import dev.photosync.core.provider.UploadRequest;
+import lombok.extern.slf4j.Slf4j;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.lang.reflect.Type;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.time.Duration;
+import java.time.Instant;
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.time.ZoneOffset;
+import java.time.format.DateTimeParseException;
+import java.util.ArrayList;
+import java.util.Base64;
+import java.util.Comparator;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Optional;
+
+/**
+ * Immich, expressed in PhotoSync's terms.
+ *
+ * <p>Two Immich design choices carry the whole browsing experience and are worth
+ * naming. Its timeline is exposed as monthly buckets with counts, so the grid
+ * knows its exact total height before fetching a single asset -- that is what
+ * makes a real virtual list possible rather than an endless scroll. And each
+ * bucket page carries a per-asset aspect ratio and thumbhash, so a tile can be
+ * laid out and filled with a recognisable blur before its image request is even
+ * queued.
+ */
+@Slf4j
+public final class ImmichProvider implements PhotoProvider {
+
+    private static final Type ALBUM_LIST = new TypeToken<List<ImmichDtos.AlbumSummary>>() {
+    }.getType();
+    private static final Type BUCKET_LIST = new TypeToken<List<ImmichDtos.TimeBucket>>() {
+    }.getType();
+    private static final Type BULK_RESULT = new TypeToken<List<ImmichDtos.BulkIdResponse>>() {
+    }.getType();
+
+    private final ProviderDescriptor descriptor;
+    private final ImmichApi api;
+
+    /**
+     * Whether this server's small thumbnails are usable, learned from the first
+     * one we fetch. See {@link #thumbnail}.
+     */
+    private volatile boolean smallThumbnails = true;
+
+    ImmichProvider(ProviderDescriptor descriptor, ProviderConnection connection) {
+        this.descriptor = descriptor;
+        this.api = new ImmichApi(connection);
+    }
+
+    @Override
+    public ProviderDescriptor descriptor() {
+        return descriptor;
+    }
+
+    @Override
+    public ProviderIdentity identify() throws ProviderException {
+        // The profile call first: a wrong API key should say "not authorised",
+        // not "could not read the version".
+        ImmichDtos.UserProfile me = api.get("/users/me", Map.of(), ImmichDtos.UserProfile.class);
+        ImmichDtos.ServerAbout about = api.get("/server/about", Map.of(), ImmichDtos.ServerAbout.class);
+        String account = Optional.ofNullable(me.name())
+                .filter(name -> !name.isBlank())
+                .orElseGet(() -> Optional.ofNullable(me.email()).orElse("unknown"));
+        return new ProviderIdentity(Optional.ofNullable(about.version()).orElse("unknown"), account);
+    }
+
+    @Override
+    public List<Album> albums() throws ProviderException {
+        List<ImmichDtos.AlbumSummary> response = api.get("/albums", Map.of(), ALBUM_LIST);
+        return response.stream()
+                .filter(summary -> summary.id() != null)
+                .map(summary -> new Album(
+                        summary.id(),
+                        Optional.ofNullable(summary.albumName()).orElse(summary.id()),
+                        Optional.ofNullable(summary.assetCount()).orElse(0),
+                        Optional.ofNullable(summary.albumThumbnailAssetId())))
+                .sorted(Comparator.comparing(Album::name, String.CASE_INSENSITIVE_ORDER))
+                .toList();
+    }
+
+    @Override
+    public Album createAlbum(String name) throws ProviderException {
+        ImmichDtos.AlbumSummary created =
+                api.post("/albums", new ImmichDtos.CreateAlbum(name), ImmichDtos.AlbumSummary.class);
+        if (created.id() == null) {
+            throw new ProviderException(ProviderException.Kind.PROTOCOL, "Immich created an album without an id");
+        }
+        return new Album(created.id(), name, 0, Optional.empty());
+    }
+
+    @Override
+    public UploadReceipt upload(UploadRequest request, TransferProgress progress) throws ProviderException {
+        Path file = request.file();
+        if (!Files.isReadable(file)) {
+            throw new ProviderException(ProviderException.Kind.SOURCE_UNREADABLE, "Cannot read " + file);
+        }
+
+        MultipartBody body;
+        try {
+            body = MultipartBody.with(file, "assetData", request.fileName(), mediaTypeOf(request.fileName()))
+                    .field("fileCreatedAt", request.capturedAt().toString())
+                    .field("fileModifiedAt", request.modifiedAt().toString())
+                    .field("filename", request.fileName())
+                    .build();
+        } catch (IOException e) {
+            throw new ProviderException(ProviderException.Kind.SOURCE_UNREADABLE, "Cannot read " + file, e);
+        }
+
+        ImmichDtos.AssetMediaResponse response =
+                api.upload("/assets", body, sha1Base64(file), progress, ImmichDtos.AssetMediaResponse.class);
+        if (response.id() == null) {
+            throw new ProviderException(ProviderException.Kind.PROTOCOL, "Immich accepted the upload without an id");
+        }
+
+        UploadReceipt.Outcome outcome = "duplicate".equalsIgnoreCase(response.status())
+                ? UploadReceipt.Outcome.DUPLICATE
+                : UploadReceipt.Outcome.CREATED;
+
+        Optional<String> album = request.album().id();
+        if (album.isPresent()) {
+            addToAlbum(album.get(), response.id());
+        }
+        return new UploadReceipt(response.id(), outcome);
+    }
+
+    @Override
+    public List<TimelineBucket> timeline(AlbumRef album) throws ProviderException {
+        List<ImmichDtos.TimeBucket> response = api.get("/timeline/buckets", scope(album), BUCKET_LIST);
+        return response.stream()
+                .filter(bucket -> bucket.timeBucket() != null)
+                .map(bucket -> monthOf(bucket.timeBucket())
+                        .map(month -> new TimelineBucket(
+                                bucket.timeBucket(), month, Math.max(0, Optional.ofNullable(bucket.count()).orElse(0)))))
+                .flatMap(Optional::stream)
+                .filter(bucket -> bucket.assetCount() > 0)
+                .sorted()
+                .toList();
+    }
+
+    @Override
+    public BucketPage page(AlbumRef album, TimelineBucket bucket) throws ProviderException {
+        Map<String, String> query = scope(album);
+        query.put("timeBucket", bucket.key());
+        ImmichDtos.BucketAssets columns = api.get("/timeline/bucket", query, ImmichDtos.BucketAssets.class);
+
+        int count = columns.size();
+        List<RemoteAsset> assets = new ArrayList<>(count);
+        for (int index = 0; index < count; index++) {
+            String id = columns.id().get(index);
+            if (id == null) {
+                continue;
+            }
+            Optional<Instant> takenAt = instantOf(column(columns.fileCreatedAt(), index));
+            if (takenAt.isEmpty()) {
+                // Without a timestamp the asset has no day to live under.
+                continue;
+            }
+            double offsetHours = column(columns.localOffsetHours(), index, 0.0);
+            boolean image = column(columns.isImage(), index, Boolean.TRUE);
+            assets.add(new RemoteAsset(
+                    id,
+                    image ? AssetKind.IMAGE : AssetKind.VIDEO,
+                    takenAt.get(),
+                    localTimeOf(takenAt.get(), offsetHours),
+                    column(columns.ratio(), index, 1.0),
+                    Optional.ofNullable(column(columns.thumbhash(), index)),
+                    Duration.ZERO));
+        }
+        assets.sort(Comparator.comparing(RemoteAsset::localCapturedAt).reversed());
+        return new BucketPage(bucket, assets);
+    }
+
+    @Override
+    public byte[] thumbnail(String assetId, ThumbnailSize size) throws ProviderException {
+        // Videos have no still of their own to serve, but Immich renders one for
+        // them at the same endpoint -- which is exactly what a video tile needs.
+        String path = "/assets/" + assetId + "/thumbnail";
+        if (size == ThumbnailSize.DETAIL || !smallThumbnails) {
+            return api.getBytes(path, Map.of("size", "preview"));
+        }
+        byte[] small = api.getBytes(path, Map.of("size", "thumbnail"));
+        if (!isWebP(small)) {
+            return small;
+        }
+        // Immich's default thumbnail format is WebP, which the game's image
+        // decoder cannot read; its previews default to JPEG, which it can. One
+        // wasted request per session buys correct tiles on those servers, and
+        // servers already configured for JPEG never take this branch.
+        log.info("This Immich server serves WebP thumbnails; falling back to previews for the grid");
+        smallThumbnails = false;
+        return api.getBytes(path, Map.of("size", "preview"));
+    }
+
+    /** RIFF container with a WEBP fourcc, per the WebP specification. */
+    private static boolean isWebP(byte[] bytes) {
+        return bytes.length >= 12
+                && bytes[0] == 'R' && bytes[1] == 'I' && bytes[2] == 'F' && bytes[3] == 'F'
+                && bytes[8] == 'W' && bytes[9] == 'E' && bytes[10] == 'B' && bytes[11] == 'P';
+    }
+
+    @Override
+    public void close() {
+        // Nothing to release: java.net.http.HttpClient has no close() before
+        // Java 21, and we compile this module to 17. It uses daemon threads and
+        // is collected with the provider.
+    }
+
+    // -----------------------------------------------------------------------
+
+    private void addToAlbum(String albumId, String assetId) throws ProviderException {
+        List<ImmichDtos.BulkIdResponse> results =
+                api.put("/albums/" + albumId + "/assets", new ImmichDtos.BulkIds(List.of(assetId)), BULK_RESULT);
+        for (ImmichDtos.BulkIdResponse result : results) {
+            boolean added = Boolean.TRUE.equals(result.success());
+            if (!added && !"duplicate".equalsIgnoreCase(result.error())) {
+                // The asset itself is safely stored, so this is a warning rather
+                // than a failure -- but the player asked for an album, so say so.
+                log.warn("Uploaded {} but Immich would not add it to album {}: {}",
+                        assetId, albumId, Optional.ofNullable(result.error()).orElse("no reason given"));
+            }
+        }
+    }
+
+    /** Album filter plus the flags that keep archived and trashed assets out of the library view. */
+    private static Map<String, String> scope(AlbumRef album) {
+        Map<String, String> query = new LinkedHashMap<>();
+        album.id().ifPresent(id -> query.put("albumId", id));
+        if (album.isLibrary()) {
+            query.put("visibility", "timeline");
+        }
+        query.put("order", "desc");
+        return query;
+    }
+
+    /**
+     * Immich has returned the bucket token as both {@code 2024-01-01} and
+     * {@code 2024-01-01T00:00:00.000Z} across releases. Only the date part is
+     * ever meaningful, and the token itself goes back to the server untouched.
+     */
+    private static Optional<LocalDate> monthOf(String bucketToken) {
+        if (bucketToken.length() < 10) {
+            return Optional.empty();
+        }
+        try {
+            return Optional.of(LocalDate.parse(bucketToken.substring(0, 10)));
+        } catch (DateTimeParseException e) {
+            log.debug("Ignoring bucket with an unparseable token: {}", bucketToken);
+            return Optional.empty();
+        }
+    }
+
+    private static Optional<Instant> instantOf(String iso) {
+        if (iso == null) {
+            return Optional.empty();
+        }
+        try {
+            return Optional.of(Instant.parse(iso));
+        } catch (DateTimeParseException e) {
+            try {
+                return Optional.of(LocalDateTime.parse(iso).toInstant(ZoneOffset.UTC));
+            } catch (DateTimeParseException ignored) {
+                return Optional.empty();
+            }
+        }
+    }
+
+    /**
+     * The wall-clock time where the photo was taken. Immich stores the capture
+     * instant in UTC alongside the offset that was in force, and some zones are
+     * on a fractional hour, so the offset is converted through seconds.
+     */
+    private static LocalDateTime localTimeOf(Instant instant, double offsetHours) {
+        double clamped = Math.max(-18.0, Math.min(18.0, offsetHours));
+        return instant.atOffset(ZoneOffset.ofTotalSeconds((int) Math.round(clamped * 3600))).toLocalDateTime();
+    }
+
+    private static <T> T column(List<T> values, int index) {
+        return values == null || index >= values.size() ? null : values.get(index);
+    }
+
+    private static <T> T column(List<T> values, int index, T fallback) {
+        T value = column(values, index);
+        return value == null ? fallback : value;
+    }
+
+    private static String mediaTypeOf(String fileName) {
+        String lower = fileName.toLowerCase(Locale.ROOT);
+        if (lower.endsWith(".png")) {
+            return "image/png";
+        }
+        if (lower.endsWith(".jpg") || lower.endsWith(".jpeg")) {
+            return "image/jpeg";
+        }
+        if (lower.endsWith(".webp")) {
+            return "image/webp";
+        }
+        return "application/octet-stream";
+    }
+
+    /**
+     * SHA-1 of the file, base64 encoded, for the {@code x-immich-checksum}
+     * header. Reading the screenshot a second time costs a millisecond and buys
+     * the server the ability to recognise a retry of an upload it already has.
+     */
+    private static String sha1Base64(Path file) throws ProviderException {
+        try (InputStream stream = Files.newInputStream(file)) {
+            MessageDigest digest = MessageDigest.getInstance("SHA-1");
+            byte[] buffer = new byte[16 * 1024];
+            int read;
+            while ((read = stream.read(buffer)) > 0) {
+                digest.update(buffer, 0, read);
+            }
+            return Base64.getEncoder().encodeToString(digest.digest());
+        } catch (IOException e) {
+            throw new ProviderException(ProviderException.Kind.SOURCE_UNREADABLE, "Cannot read " + file, e);
+        } catch (NoSuchAlgorithmException e) {
+            throw new IllegalStateException("Every JVM is required to provide SHA-1", e);
+        }
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/immich/ImmichProviderFactory.java

@@ -0,0 +1,29 @@
+package dev.photosync.core.provider.immich;
+
+import dev.photosync.core.provider.PhotoProvider;
+import dev.photosync.core.provider.ProviderConnection;
+import dev.photosync.core.provider.ProviderDescriptor;
+import dev.photosync.core.provider.ProviderFactory;
+import dev.photosync.core.provider.ProviderId;
+
+/**
+ * Registers Immich with the {@code ProviderCatalog}.
+ *
+ * <p>The one place in the mod where the string "immich" appears outside a
+ * language file. A second backend is a second class like this one.
+ */
+public final class ImmichProviderFactory implements ProviderFactory {
+
+    private final ProviderDescriptor descriptor =
+            new ProviderDescriptor(new ProviderId("immich"), "photosync.provider.immich", true);
+
+    @Override
+    public ProviderDescriptor descriptor() {
+        return descriptor;
+    }
+
+    @Override
+    public PhotoProvider connect(ProviderConnection connection) {
+        return new ImmichProvider(descriptor, connection);
+    }
+}

shared/core/src/main/java/dev/photosync/core/provider/immich/MultipartBody.java

@@ -0,0 +1,178 @@
+package dev.photosync.core.provider.immich;
+
+import dev.photosync.core.provider.TransferProgress;
+
+import java.io.ByteArrayInputStream;
+import java.io.FilterInputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.SequenceInputStream;
+import java.io.UncheckedIOException;
+import java.net.http.HttpRequest;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.UUID;
+
+/**
+ * A {@code multipart/form-data} body that streams one file from disk.
+ *
+ * <p>The obvious implementation -- read the screenshot into a byte array and
+ * hand it to {@code BodyPublishers.ofByteArray} -- would double the mod's peak
+ * memory for every concurrent upload, on a heap the game is already competing
+ * for. So the body is assembled as a preamble, the file, and an epilogue, and
+ * the file part is never held in memory.
+ *
+ * <p>The length is computed up front so the request can carry a real
+ * {@code Content-Length}. Streaming with an unknown length would force chunked
+ * transfer encoding, which self-hosted reverse proxies in front of Immich are
+ * not reliably configured for.
+ */
+final class MultipartBody {
+
+    /** Report progress at most this often, so a fast local server is not drowned in callbacks. */
+    private static final int PROGRESS_STRIDE_BYTES = 64 * 1024;
+
+    private final String boundary;
+    private final byte[] preamble;
+    private final byte[] epilogue;
+    private final Path file;
+    private final long fileSize;
+
+    private MultipartBody(String boundary, byte[] preamble, byte[] epilogue, Path file, long fileSize) {
+        this.boundary = boundary;
+        this.preamble = preamble;
+        this.epilogue = epilogue;
+        this.file = file;
+        this.fileSize = fileSize;
+    }
+
+    static Builder with(Path file, String fieldName, String fileName, String contentType) throws IOException {
+        return new Builder(file, fieldName, fileName, contentType);
+    }
+
+    String contentType() {
+        return "multipart/form-data; boundary=" + boundary;
+    }
+
+    long contentLength() {
+        return preamble.length + fileSize + epilogue.length;
+    }
+
+    HttpRequest.BodyPublisher publisher(TransferProgress progress) {
+        long total = contentLength();
+        return HttpRequest.BodyPublishers.fromPublisher(
+                HttpRequest.BodyPublishers.ofInputStream(() -> open(progress, total)),
+                total);
+    }
+
+    private InputStream open(TransferProgress progress, long total) {
+        try {
+            InputStream body = new SequenceInputStream(Collections.enumeration(List.<InputStream>of(
+                    new ByteArrayInputStream(preamble),
+                    Files.newInputStream(file),
+                    new ByteArrayInputStream(epilogue))));
+            return new CountingStream(body, progress, total);
+        } catch (IOException e) {
+            // The supplier cannot throw a checked exception; the HttpClient
+            // unwraps this back into an IOException on send().
+            throw new UncheckedIOException(e);
+        }
+    }
+
+    /**
+     * Counts bytes as the HTTP client pulls them, and tells the queue.
+     *
+     * <p>The callback may throw -- that is how a cancelled upload unwinds -- so
+     * it is invoked where the exception can propagate out of {@code read}.
+     */
+    private static final class CountingStream extends FilterInputStream {
+
+        private final TransferProgress progress;
+        private final long total;
+        private long sent;
+        private long lastReported;
+
+        CountingStream(InputStream delegate, TransferProgress progress, long total) {
+            super(delegate);
+            this.progress = progress;
+            this.total = total;
+        }
+
+        @Override
+        public int read() throws IOException {
+            int value = super.read();
+            if (value >= 0) {
+                advance(1);
+            }
+            return value;
+        }
+
+        @Override
+        public int read(byte[] buffer, int offset, int length) throws IOException {
+            int count = super.read(buffer, offset, length);
+            if (count > 0) {
+                advance(count);
+            }
+            return count;
+        }
+
+        private void advance(int count) {
+            sent += count;
+            if (sent - lastReported >= PROGRESS_STRIDE_BYTES || sent == total) {
+                lastReported = sent;
+                progress.onProgress(sent, total);
+            }
+        }
+    }
+
+    static final class Builder {
+
+        private final String boundary = "PhotoSync" + UUID.randomUUID().toString().replace("-", "");
+        private final List<String> fields = new ArrayList<>();
+        private final Path file;
+        private final String fieldName;
+        private final String fileName;
+        private final String contentType;
+        private final long fileSize;
+
+        private Builder(Path file, String fieldName, String fileName, String contentType) throws IOException {
+            this.file = file;
+            this.fieldName = fieldName;
+            this.fileName = fileName;
+            this.contentType = contentType;
+            this.fileSize = Files.size(file);
+        }
+
+        Builder field(String name, String value) {
+            fields.add("--" + boundary + "\r\n"
+                    + "Content-Disposition: form-data; name=\"" + name + "\"\r\n\r\n"
+                    + value + "\r\n");
+            return this;
+        }
+
+        MultipartBody build() {
+            StringBuilder head = new StringBuilder();
+            fields.forEach(head::append);
+            head.append("--").append(boundary).append("\r\n")
+                    .append("Content-Disposition: form-data; name=\"").append(fieldName)
+                    .append("\"; filename=\"").append(escape(fileName)).append("\"\r\n")
+                    .append("Content-Type: ").append(contentType).append("\r\n\r\n");
+
+            return new MultipartBody(
+                    boundary,
+                    head.toString().getBytes(StandardCharsets.UTF_8),
+                    ("\r\n--" + boundary + "--\r\n").getBytes(StandardCharsets.UTF_8),
+                    file,
+                    fileSize);
+        }
+
+        /** Quotes and backslashes in a file name would end the header early. */
+        private static String escape(String raw) {
+            return raw.replace("\\", "\\\\").replace("\"", "\\\"").replace("\r", "").replace("\n", "");
+        }
+    }
+}

shared/core/src/main/java/dev/photosync/core/thumbnail/ThumbHash.java

@@ -0,0 +1,172 @@
+package dev.photosync.core.thumbnail;
+
+import java.util.Arrays;
+import java.util.Base64;
+import java.util.Optional;
+
+/**
+ * Decoder for the ThumbHash placeholders Immich attaches to every asset.
+ *
+ * <p>A ThumbHash is around 25 bytes and already travels with the timeline
+ * response, so the blurred stand-in for a tile costs no extra request -- the
+ * grid can be fully painted the instant a month's metadata lands, and the real
+ * thumbnails fade in behind it. That is what makes fast scrolling feel like the
+ * web gallery this browser is imitating rather than a wall of grey boxes.
+ *
+ * <p>Only decoding lives here; the mod never produces hashes. The format is
+ * Evan Wallace's, and the maths below deliberately mirrors the reference
+ * implementation step for step so it can be diffed against it.
+ */
+public final class ThumbHash {
+
+    /** The reference decoder's output is normalised to fit in a 32x32 box. */
+    private static final int MAX_EDGE = 32;
+
+    private ThumbHash() {
+    }
+
+    /** Decodes the base64 form carried in the timeline response. */
+    public static Optional<ThumbImage> decode(String base64) {
+        if (base64 == null || base64.isBlank()) {
+            return Optional.empty();
+        }
+        try {
+            return decode(Base64.getDecoder().decode(base64.trim()));
+        } catch (IllegalArgumentException e) {
+            return Optional.empty();
+        }
+    }
+
+    /**
+     * Decodes the raw hash.
+     *
+     * <p>Returns empty rather than throwing for anything malformed: a bad
+     * placeholder should cost the player a grey tile, never a crashed screen.
+     */
+    public static Optional<ThumbImage> decode(byte[] hash) {
+        if (hash == null || hash.length < 5) {
+            return Optional.empty();
+        }
+        try {
+            return Optional.of(decodeChecked(hash));
+        } catch (ArrayIndexOutOfBoundsException e) {
+            return Optional.empty();
+        }
+    }
+
+    private static ThumbImage decodeChecked(byte[] hash) {
+        int header24 = byteAt(hash, 0) | (byteAt(hash, 1) << 8) | (byteAt(hash, 2) << 16);
+        int header16 = byteAt(hash, 3) | (byteAt(hash, 4) << 8);
+
+        float lDc = (header24 & 63) / 63f;
+        float pDc = ((header24 >> 6) & 63) / 31.5f - 1f;
+        float qDc = ((header24 >> 12) & 63) / 31.5f - 1f;
+        float lScale = ((header24 >> 18) & 31) / 31f;
+        boolean hasAlpha = (header24 >>> 23) != 0;
+        float pScale = ((header16 >> 3) & 63) / 63f;
+        float qScale = ((header16 >> 9) & 63) / 63f;
+        boolean landscape = (header16 >>> 15) != 0;
+
+        int lx = Math.max(3, landscape ? (hasAlpha ? 5 : 7) : (header16 & 7));
+        int ly = Math.max(3, landscape ? (header16 & 7) : (hasAlpha ? 5 : 7));
+
+        float aDc = hasAlpha ? (byteAt(hash, 5) & 15) / 15f : 1f;
+        float aScale = hasAlpha ? (byteAt(hash, 5) >> 4) / 15f : 0f;
+
+        // Saturation is boosted by 1.25 on the chroma channels to undo the loss
+        // from quantising them into four bits, exactly as the encoder expects.
+        int[] cursor = {0};
+        int acStart = hasAlpha ? 6 : 5;
+        float[] lAc = readChannel(hash, acStart, cursor, lx, ly, lScale);
+        float[] pAc = readChannel(hash, acStart, cursor, 3, 3, pScale * 1.25f);
+        float[] qAc = readChannel(hash, acStart, cursor, 3, 3, qScale * 1.25f);
+        float[] aAc = hasAlpha ? readChannel(hash, acStart, cursor, 5, 5, aScale) : new float[0];
+
+        // The aspect ratio comes from the unclamped counts, matching the encoder.
+        float ratio = (float) (landscape ? (hasAlpha ? 5 : 7) : (header16 & 7))
+                / (landscape ? (header16 & 7) : (hasAlpha ? 5 : 7));
+        int width = Math.max(1, Math.round(ratio > 1 ? MAX_EDGE : MAX_EDGE * ratio));
+        int height = Math.max(1, Math.round(ratio > 1 ? MAX_EDGE / ratio : MAX_EDGE));
+
+        // The reference recomputes these cosines per pixel; hoisting them out
+        // turns the inner loop into pure multiply-add.
+        int fxCount = Math.max(lx, hasAlpha ? 5 : 3);
+        int fyCount = Math.max(ly, hasAlpha ? 5 : 3);
+        float[][] fxTable = cosineTable(width, fxCount);
+        float[][] fyTable = cosineTable(height, fyCount);
+
+        int[] argb = new int[width * height];
+        for (int y = 0; y < height; y++) {
+            float[] fy = fyTable[y];
+            for (int x = 0; x < width; x++) {
+                float[] fx = fxTable[x];
+                float l = lDc;
+                float p = pDc;
+                float q = qDc;
+                float a = aDc;
+
+                for (int cy = 0, j = 0; cy < ly; cy++) {
+                    for (int cx = cy > 0 ? 0 : 1; cx * ly < lx * (ly - cy); cx++, j++) {
+                        l += lAc[j] * fx[cx] * fy[cy] * 2f;
+                    }
+                }
+                for (int cy = 0, j = 0; cy < 3; cy++) {
+                    for (int cx = cy > 0 ? 0 : 1; cx < 3 - cy; cx++, j++) {
+                        float f = fx[cx] * fy[cy] * 2f;
+                        p += pAc[j] * f;
+                        q += qAc[j] * f;
+                    }
+                }
+                if (hasAlpha) {
+                    for (int cy = 0, j = 0; cy < 5; cy++) {
+                        for (int cx = cy > 0 ? 0 : 1; cx < 5 - cy; cx++, j++) {
+                            a += aAc[j] * fx[cx] * fy[cy] * 2f;
+                        }
+                    }
+                }
+
+                float blue = l - 2f / 3f * p;
+                float red = (3f * l - blue + q) / 2f;
+                float green = red - q;
+                argb[y * width + x] = (channel(a) << 24) | (channel(red) << 16)
+                        | (channel(green) << 8) | channel(blue);
+            }
+        }
+        return new ThumbImage(width, height, argb);
+    }
+
+    /**
+     * Reads one channel's AC coefficients, four bits each, from a nibble stream
+     * shared by all channels -- hence the caller-owned cursor.
+     */
+    private static float[] readChannel(byte[] hash, int start, int[] cursor, int nx, int ny, float scale) {
+        float[] ac = new float[nx * ny];
+        int count = 0;
+        for (int cy = 0; cy < ny; cy++) {
+            for (int cx = cy > 0 ? 0 : 1; cx * ny < nx * (ny - cy); cx++) {
+                int index = cursor[0]++;
+                int nibble = (byteAt(hash, start + (index >> 1)) >> ((index & 1) << 2)) & 15;
+                ac[count++] = (nibble / 7.5f - 1f) * scale;
+            }
+        }
+        return Arrays.copyOf(ac, count);
+    }
+
+    private static float[][] cosineTable(int size, int terms) {
+        float[][] table = new float[size][terms];
+        for (int i = 0; i < size; i++) {
+            for (int c = 0; c < terms; c++) {
+                table[i][c] = (float) Math.cos(Math.PI / size * (i + 0.5) * c);
+            }
+        }
+        return table;
+    }
+
+    private static int channel(float value) {
+        return Math.max(0, Math.min(255, Math.round(value * 255f)));
+    }
+
+    private static int byteAt(byte[] hash, int index) {
+        return hash[index] & 0xFF;
+    }
+}

shared/core/src/main/java/dev/photosync/core/thumbnail/ThumbImage.java

@@ -0,0 +1,25 @@
+package dev.photosync.core.thumbnail;
+
+/**
+ * A tiny decoded bitmap -- at most 32x32 -- in 0xAARRGGBB order.
+ *
+ * <p>Deliberately not a Minecraft {@code NativeImage}: this module has no
+ * graphics dependencies, and the version adapters are the ones that know how to
+ * turn a pixel array into a texture on their Minecraft version.
+ */
+public record ThumbImage(int width, int height, int[] argb) {
+
+    public ThumbImage {
+        argb = argb.clone();
+    }
+
+    public int pixel(int x, int y) {
+        return argb[y * width + x];
+    }
+
+    /** A defensive copy, because callers hand this straight to native buffers. */
+    @Override
+    public int[] argb() {
+        return argb.clone();
+    }
+}

shared/core/src/main/java/dev/photosync/core/thumbnail/ThumbnailLoader.java

@@ -0,0 +1,121 @@
+package dev.photosync.core.thumbnail;
+
+import dev.photosync.core.provider.PhotoProvider;
+import dev.photosync.core.provider.ProviderException;
+import dev.photosync.core.provider.ProviderSession;
+import dev.photosync.core.provider.ThumbnailSize;
+import lombok.extern.slf4j.Slf4j;
+
+import java.util.Map;
+import java.util.Optional;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ThreadFactory;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+
+/**
+ * Fetches thumbnail bytes off the render thread, one request per asset no matter
+ * how many tiles ask for it.
+ *
+ * <p>Decoding those bytes into a texture is deliberately <em>not</em> done here:
+ * that has to happen on the render thread against a version-specific image type,
+ * so the caller polls the returned future and uploads the result itself.
+ *
+ * <p>The loader refuses work once too many requests are outstanding. During a
+ * fast scroll most tiles are on screen for a few frames and their thumbnails
+ * would arrive long after the player has moved past them; dropping those
+ * requests and letting the grid ask again when it settles keeps the queue short
+ * and the visible tiles first in line.
+ */
+@Slf4j
+public final class ThumbnailLoader implements AutoCloseable {
+
+    /** Roughly two screens' worth of tiles. Past this, the queue is stale by the time it drains. */
+    private static final int MAX_PENDING = 96;
+    private static final int WORKERS = 3;
+
+    private final ProviderSession session;
+    private final ExecutorService workers;
+    private final Map<String, CompletableFuture<byte[]>> inFlight = new ConcurrentHashMap<>();
+
+    public ThumbnailLoader(ProviderSession session) {
+        this.session = session;
+        this.workers = Executors.newFixedThreadPool(WORKERS, daemonFactory());
+    }
+
+    /**
+     * Asks for one thumbnail.
+     *
+     * <p>Returns empty when the loader is saturated, which is a "not now" rather
+     * than a failure -- the caller is expected to try again on a later frame.
+     * Two callers asking for the same asset share one request and one future.
+     */
+    public Optional<CompletableFuture<byte[]>> request(String assetId, ThumbnailSize size) {
+        String key = assetId + '@' + size;
+        CompletableFuture<byte[]> existing = inFlight.get(key);
+        if (existing != null) {
+            return Optional.of(existing);
+        }
+        if (inFlight.size() >= MAX_PENDING) {
+            return Optional.empty();
+        }
+        CompletableFuture<byte[]> future = new CompletableFuture<>();
+        CompletableFuture<byte[]> raced = inFlight.putIfAbsent(key, future);
+        if (raced != null) {
+            return Optional.of(raced);
+        }
+        try {
+            workers.execute(() -> {
+                try {
+                    future.complete(fetch(assetId, size));
+                } catch (ProviderException e) {
+                    log.debug("Thumbnail {} failed: {}", assetId, e.getMessage());
+                    future.completeExceptionally(e);
+                } catch (RuntimeException e) {
+                    future.completeExceptionally(e);
+                } finally {
+                    inFlight.remove(key);
+                }
+            });
+        } catch (RuntimeException e) {
+            // Shutting down: the screen is closing and nobody will read this.
+            inFlight.remove(key);
+            future.completeExceptionally(e);
+        }
+        return Optional.of(future);
+    }
+
+    public int pending() {
+        return inFlight.size();
+    }
+
+    private byte[] fetch(String assetId, ThumbnailSize size) throws ProviderException {
+        PhotoProvider provider = session.provider().orElseThrow(() -> new ProviderException(
+                ProviderException.Kind.AUTHENTICATION, "No provider is configured"));
+        return provider.thumbnail(assetId, size);
+    }
+
+    @Override
+    public void close() {
+        workers.shutdownNow();
+        inFlight.values().forEach(future -> future.cancel(false));
+        inFlight.clear();
+        try {
+            workers.awaitTermination(2, TimeUnit.SECONDS);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        }
+    }
+
+    private static ThreadFactory daemonFactory() {
+        AtomicInteger counter = new AtomicInteger();
+        return runnable -> {
+            Thread thread = new Thread(runnable, "photosync-thumbnail-" + counter.incrementAndGet());
+            thread.setDaemon(true);
+            return thread;
+        };
+    }
+}

shared/core/src/main/java/dev/photosync/core/timeline/TimelineBrowser.java

@@ -0,0 +1,282 @@
+package dev.photosync.core.timeline;
+
+import dev.photosync.core.provider.AlbumRef;
+import dev.photosync.core.provider.BucketPage;
+import dev.photosync.core.provider.PhotoProvider;
+import dev.photosync.core.provider.ProviderException;
+import dev.photosync.core.provider.ProviderSession;
+import dev.photosync.core.provider.RemoteAsset;
+import dev.photosync.core.provider.TimelineBucket;
+import lombok.extern.slf4j.Slf4j;
+
+import java.time.LocalDate;
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ThreadFactory;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+
+/**
+ * The lazily-filled model behind the album browser.
+ *
+ * <p>It fetches the month counts once, which is enough to lay the whole album
+ * out at full height, and then fetches a month's assets only when the renderer
+ * says that month is about to be on screen. Everything here runs off the render
+ * thread; the renderer only ever reads {@link #sections()} and {@link #revision()},
+ * both of which are non-blocking.
+ *
+ * <p>The renderer drives loading by calling {@link #request(TimelineBucket)} for
+ * the months it is about to draw. Repeat calls are free -- already-loaded,
+ * in-flight and previously-failed months are all ignored -- so it can call it
+ * every frame without keeping any bookkeeping of its own.
+ */
+@Slf4j
+public final class TimelineBrowser implements AutoCloseable {
+
+    private final ProviderSession session;
+    private final ExecutorService loader;
+    private final Object lock = new Object();
+    private final Set<String> inFlight = ConcurrentHashMap.newKeySet();
+    private final AtomicInteger revision = new AtomicInteger();
+
+    private final Map<String, List<TimelineSection.Day>> pages = new HashMap<>();
+    private final Map<String, String> pageErrors = new HashMap<>();
+
+    private AlbumRef album = AlbumRef.library();
+    private List<TimelineBucket> buckets = List.of();
+    /** Bumped on every {@link #open} so results for the previous album are discarded. */
+    private long generation;
+
+    private volatile List<TimelineSection> sections = List.of();
+    private volatile TimelineState state = TimelineState.NOT_CONFIGURED;
+    private volatile String error;
+
+    public TimelineBrowser(ProviderSession session) {
+        this.session = session;
+        this.loader = Executors.newFixedThreadPool(2, daemonFactory());
+    }
+
+    /** Points the browser at an album (or the whole library) and starts loading. */
+    public void open(AlbumRef target) {
+        long token;
+        synchronized (lock) {
+            album = target;
+            buckets = List.of();
+            pages.clear();
+            pageErrors.clear();
+            sections = List.of();
+            error = null;
+            token = ++generation;
+        }
+        inFlight.clear();
+        state = TimelineState.LOADING;
+        revision.incrementAndGet();
+        loader.execute(() -> loadBuckets(token));
+    }
+
+    /** Re-fetches everything for the album already open -- the refresh button. */
+    public void reload() {
+        AlbumRef target;
+        synchronized (lock) {
+            target = album;
+        }
+        open(target);
+    }
+
+    /**
+     * The current layout: one entry per calendar day for loaded months, one per
+     * month for the rest. Cheap enough to call every frame -- it is a cached
+     * immutable list, rebuilt only when something actually arrives.
+     */
+    public List<TimelineSection> sections() {
+        return sections;
+    }
+
+    /** Changes whenever {@link #sections()} does, so the renderer knows to re-measure. */
+    public int revision() {
+        return revision.get();
+    }
+
+    public TimelineState state() {
+        return state;
+    }
+
+    public Optional<String> error() {
+        return Optional.ofNullable(error);
+    }
+
+    /** Total assets across the album, known before anything is fetched. */
+    public int assetCount() {
+        synchronized (lock) {
+            return buckets.stream().mapToInt(TimelineBucket::assetCount).sum();
+        }
+    }
+
+    /** Asks for a month's assets. Loaded, loading and failed months are no-ops. */
+    public void request(TimelineBucket bucket) {
+        long token;
+        synchronized (lock) {
+            if (pages.containsKey(bucket.key()) || pageErrors.containsKey(bucket.key())) {
+                return;
+            }
+            token = generation;
+        }
+        if (!inFlight.add(bucket.key())) {
+            return;
+        }
+        loader.execute(() -> loadPage(token, bucket));
+    }
+
+    /** Why a month's tiles are still blank, if it failed rather than merely not being reached yet. */
+    public Optional<String> pageError(TimelineBucket bucket) {
+        synchronized (lock) {
+            return Optional.ofNullable(pageErrors.get(bucket.key()));
+        }
+    }
+
+    public boolean isLoading(TimelineBucket bucket) {
+        return inFlight.contains(bucket.key());
+    }
+
+    /** Clears a month's recorded failure so the next {@link #request} tries again. */
+    public void retryPage(TimelineBucket bucket) {
+        synchronized (lock) {
+            pageErrors.remove(bucket.key());
+        }
+        request(bucket);
+    }
+
+    @Override
+    public void close() {
+        loader.shutdownNow();
+        try {
+            loader.awaitTermination(2, TimeUnit.SECONDS);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        }
+    }
+
+    private void loadBuckets(long token) {
+        Optional<PhotoProvider> provider = session.provider();
+        if (provider.isEmpty()) {
+            state = TimelineState.NOT_CONFIGURED;
+            revision.incrementAndGet();
+            return;
+        }
+        AlbumRef target;
+        synchronized (lock) {
+            if (token != generation) {
+                return;
+            }
+            target = album;
+        }
+        try {
+            List<TimelineBucket> fetched = new ArrayList<>(provider.get().timeline(target));
+            fetched.sort(Comparator.naturalOrder());
+            synchronized (lock) {
+                if (token != generation) {
+                    return;
+                }
+                buckets = List.copyOf(fetched);
+                rebuild();
+            }
+            state = fetched.isEmpty() ? TimelineState.EMPTY : TimelineState.READY;
+        } catch (ProviderException e) {
+            log.warn("Could not load the timeline: {}", e.getMessage());
+            synchronized (lock) {
+                if (token != generation) {
+                    return;
+                }
+                error = e.getMessage();
+            }
+            state = TimelineState.FAILED;
+        } finally {
+            revision.incrementAndGet();
+        }
+    }
+
+    private void loadPage(long token, TimelineBucket bucket) {
+        try {
+            Optional<PhotoProvider> provider = session.provider();
+            if (provider.isEmpty()) {
+                return;
+            }
+            AlbumRef target;
+            synchronized (lock) {
+                if (token != generation) {
+                    return;
+                }
+                target = album;
+            }
+            BucketPage page = provider.get().page(target, bucket);
+            List<TimelineSection.Day> days = groupByDay(page.assets());
+            synchronized (lock) {
+                if (token != generation) {
+                    return;
+                }
+                pages.put(bucket.key(), days);
+                rebuild();
+            }
+        } catch (ProviderException e) {
+            log.warn("Could not load {}: {}", bucket.key(), e.getMessage());
+            synchronized (lock) {
+                if (token == generation) {
+                    pageErrors.put(bucket.key(), e.getMessage());
+                }
+            }
+        } finally {
+            inFlight.remove(bucket.key());
+            revision.incrementAndGet();
+        }
+    }
+
+    /**
+     * Splits a month into days, newest first.
+     *
+     * <p>Grouping is by the asset's <em>local</em> date -- the one the camera saw
+     * -- so a photo taken at 23:00 stays on the day it was taken no matter which
+     * timezone the player is browsing from.
+     */
+    private static List<TimelineSection.Day> groupByDay(List<RemoteAsset> assets) {
+        Map<LocalDate, List<RemoteAsset>> byDay = new LinkedHashMap<>();
+        for (RemoteAsset asset : assets) {
+            byDay.computeIfAbsent(asset.localDay(), day -> new ArrayList<>()).add(asset);
+        }
+        List<TimelineSection.Day> days = new ArrayList<>(byDay.size());
+        byDay.forEach((day, members) -> days.add(new TimelineSection.Day(day, members)));
+        days.sort(Comparator.comparing(TimelineSection.Day::date).reversed());
+        return List.copyOf(days);
+    }
+
+    /** Must hold {@link #lock}. */
+    private void rebuild() {
+        List<TimelineSection> built = new ArrayList<>();
+        for (TimelineBucket bucket : buckets) {
+            List<TimelineSection.Day> loaded = pages.get(bucket.key());
+            if (loaded == null) {
+                built.add(new TimelineSection.PendingMonth(bucket));
+            } else {
+                built.addAll(loaded);
+            }
+        }
+        sections = List.copyOf(built);
+    }
+
+    private static ThreadFactory daemonFactory() {
+        AtomicInteger counter = new AtomicInteger();
+        return runnable -> {
+            Thread thread = new Thread(runnable, "photosync-timeline-" + counter.incrementAndGet());
+            thread.setDaemon(true);
+            return thread;
+        };
+    }
+}

shared/core/src/main/java/dev/photosync/core/timeline/TimelineSection.java

@@ -0,0 +1,61 @@
+package dev.photosync.core.timeline;
+
+import dev.photosync.core.provider.RemoteAsset;
+import dev.photosync.core.provider.TimelineBucket;
+
+import java.time.LocalDate;
+import java.util.List;
+
+/**
+ * One labelled run of tiles in the browser.
+ *
+ * <p>The backend groups by month; the player asked to browse by day. Those two
+ * facts are reconciled here rather than in the renderer: a month that has not
+ * been fetched yet contributes a single {@link PendingMonth} whose asset count
+ * is known exactly, and the moment its page arrives it is replaced by one
+ * {@link Day} per calendar day inside it.
+ *
+ * <p>Because the count is exact either way, the list has its true height before
+ * anything is loaded -- so the scrollbar is honest from the first frame and the
+ * browser only ever fetches the months the viewport actually reaches.
+ */
+public sealed interface TimelineSection {
+
+    /** The day, or the first of the month for a section that is still pending. */
+    LocalDate date();
+
+    int assetCount();
+
+    /** Assets known so far. Empty while pending, which is what makes a tile a placeholder. */
+    List<RemoteAsset> assets();
+
+    record PendingMonth(TimelineBucket bucket) implements TimelineSection {
+
+        @Override
+        public LocalDate date() {
+            return bucket.month();
+        }
+
+        @Override
+        public int assetCount() {
+            return bucket.assetCount();
+        }
+
+        @Override
+        public List<RemoteAsset> assets() {
+            return List.of();
+        }
+    }
+
+    record Day(LocalDate date, List<RemoteAsset> assets) implements TimelineSection {
+
+        public Day {
+            assets = List.copyOf(assets);
+        }
+
+        @Override
+        public int assetCount() {
+            return assets.size();
+        }
+    }
+}

shared/core/src/main/java/dev/photosync/core/timeline/TimelineState.java

@@ -0,0 +1,14 @@
+package dev.photosync.core.timeline;
+
+/** What the browser should be showing instead of, or alongside, the grid. */
+public enum TimelineState {
+    /** No credentials yet. The settings screen is the thing to offer. */
+    NOT_CONFIGURED,
+    /** Fetching the bucket list. Nothing can be laid out yet. */
+    LOADING,
+    READY,
+    /** The bucket list failed; {@code TimelineBrowser.error()} says why. */
+    FAILED,
+    /** The album really is empty. */
+    EMPTY
+}

shared/core/src/main/java/dev/photosync/core/upload/QueuedUpload.java

@@ -0,0 +1,20 @@
+package dev.photosync.core.upload;
+
+/**
+ * What the GUI draws for one row: the job, plus the byte counters that are not
+ * worth persisting and would be stale a frame later anyway.
+ */
+public record QueuedUpload(UploadJob job, long bytesSent, long bytesTotal) {
+
+    public static QueuedUpload idle(UploadJob job) {
+        return new QueuedUpload(job, 0L, job.sizeBytes());
+    }
+
+    /** 0..1, and 0 rather than NaN when the total is not known yet. */
+    public double fraction() {
+        if (bytesTotal <= 0L) {
+            return 0.0;
+        }
+        return Math.min(1.0, (double) bytesSent / (double) bytesTotal);
+    }
+}

shared/core/src/main/java/dev/photosync/core/upload/UploadCoordinator.java

@@ -0,0 +1,191 @@
+package dev.photosync.core.upload;
+
+import dev.photosync.core.config.UploadSettings;
+import dev.photosync.core.provider.PhotoProvider;
+import dev.photosync.core.provider.ProviderException;
+import dev.photosync.core.provider.ProviderSession;
+import dev.photosync.core.provider.TransferCancelledException;
+import dev.photosync.core.provider.UploadReceipt;
+import lombok.extern.slf4j.Slf4j;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.time.Duration;
+import java.time.Instant;
+import java.util.Optional;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ThreadFactory;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.Supplier;
+
+/**
+ * Drains the {@link UploadQueue}.
+ *
+ * <p>A single pump thread wakes a few times a second, works out how many slots
+ * the current settings allow, and hands that many due jobs to a small worker
+ * pool. Nothing has to be reconfigured when the player changes the concurrency:
+ * the next tick simply reads the new number. The same is true of credentials --
+ * while none are set the pump finds no provider and the queue just waits, which
+ * is exactly the behaviour a first-run player wants.
+ *
+ * <p>Threads are daemons. Correctness across a hard shutdown comes from the
+ * queue being durable, not from holding the JVM open; blocking exit on a stuck
+ * socket would be a much worse bug than re-uploading one file.
+ */
+@Slf4j
+public final class UploadCoordinator implements AutoCloseable {
+
+    private static final Duration PUMP_INTERVAL = Duration.ofMillis(250);
+    private static final Duration MAX_BACKOFF = Duration.ofMinutes(10);
+
+    private final UploadQueue queue;
+    private final ProviderSession session;
+    private final Supplier<UploadSettings> settings;
+
+    private final ScheduledExecutorService pump;
+    private final ExecutorService workers;
+    private final AtomicInteger inFlight = new AtomicInteger();
+
+    private volatile boolean running;
+
+    public UploadCoordinator(UploadQueue queue, ProviderSession session, Supplier<UploadSettings> settings) {
+        this.queue = queue;
+        this.session = session;
+        this.settings = settings;
+        this.pump = Executors.newSingleThreadScheduledExecutor(named("photosync-pump"));
+        this.workers = Executors.newFixedThreadPool(UploadSettings.MAX_CONCURRENCY, named("photosync-upload"));
+    }
+
+    public void start() {
+        running = true;
+        pump.scheduleWithFixedDelay(this::tick, 0, PUMP_INTERVAL.toMillis(), TimeUnit.MILLISECONDS);
+    }
+
+    /** Nudges the pump so a just-taken screenshot does not sit for a quarter of a second. */
+    public void wake() {
+        if (running) {
+            pump.execute(this::tick);
+        }
+    }
+
+    /**
+     * Stops taking new work and waits for the transfers already in flight.
+     *
+     * @return true if everything finished within the timeout
+     */
+    public boolean shutdown(Duration timeout) {
+        running = false;
+        pump.shutdown();
+        workers.shutdown();
+        try {
+            return workers.awaitTermination(timeout.toMillis(), TimeUnit.MILLISECONDS);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+            return false;
+        }
+    }
+
+    @Override
+    public void close() {
+        if (!shutdown(Duration.ofSeconds(5))) {
+            // Whatever is still running will be re-tried next launch: the queue
+            // file already says these jobs were interrupted.
+            workers.shutdownNow();
+            pump.shutdownNow();
+        }
+    }
+
+    // -----------------------------------------------------------------------
+
+    private void tick() {
+        if (!running) {
+            return;
+        }
+        Optional<PhotoProvider> provider = session.provider();
+        if (provider.isEmpty()) {
+            return;
+        }
+        UploadSettings current = settings.get();
+        Instant now = Instant.now();
+        while (running && inFlight.get() < current.concurrency()) {
+            Optional<UploadJob> claimed = queue.claim(now);
+            if (claimed.isEmpty()) {
+                return;
+            }
+            inFlight.incrementAndGet();
+            UploadJob job = claimed.get();
+            try {
+                workers.execute(() -> transfer(job, provider.get(), current));
+            } catch (RuntimeException e) {
+                // Pool already shutting down; put the job back for next launch.
+                inFlight.decrementAndGet();
+                queue.reschedule(job.id(), "Shutting down", now);
+                return;
+            }
+        }
+    }
+
+    private void transfer(UploadJob job, PhotoProvider provider, UploadSettings current) {
+        try {
+            UploadReceipt receipt = provider.upload(job.toRequest(), (sent, total) -> {
+                if (queue.isCancelRequested(job.id())) {
+                    throw new TransferCancelledException(job.id());
+                }
+                queue.reportProgress(job.id(), sent, total);
+            });
+            queue.succeed(job.id(), receipt);
+            if (current.deleteLocalAfterUpload()) {
+                removeLocalCopy(job);
+            }
+        } catch (TransferCancelledException e) {
+            log.debug("Upload of {} cancelled by the player", job.fileName());
+        } catch (ProviderException e) {
+            handleFailure(job, current, e);
+        } catch (RuntimeException e) {
+            log.error("Unexpected error uploading {}", job.fileName(), e);
+            queue.fail(job.id(), e.toString());
+        } finally {
+            inFlight.decrementAndGet();
+        }
+    }
+
+    private void handleFailure(UploadJob job, UploadSettings current, ProviderException failure) {
+        String message = failure.getMessage() == null ? failure.kind().name() : failure.getMessage();
+        boolean tryAgain = failure.isRetryable() && job.attempts() < current.maxAttempts();
+        if (tryAgain) {
+            Duration wait = backoff(job.attempts(), current);
+            log.info("Upload of {} failed ({}); retrying in {}s", job.fileName(), message, wait.toSeconds());
+            queue.reschedule(job.id(), message, Instant.now().plus(wait));
+        } else {
+            log.warn("Giving up on {}: {}", job.fileName(), message);
+            queue.fail(job.id(), message);
+        }
+    }
+
+    /** Exponential, so a server that is down for a while stops being hammered. */
+    private static Duration backoff(int attempts, UploadSettings current) {
+        int exponent = Math.min(attempts - 1, 16);
+        long seconds = (long) current.retryBackoffSeconds() << exponent;
+        return seconds >= MAX_BACKOFF.toSeconds() ? MAX_BACKOFF : Duration.ofSeconds(seconds);
+    }
+
+    private static void removeLocalCopy(UploadJob job) {
+        try {
+            Files.deleteIfExists(job.path());
+        } catch (IOException e) {
+            log.warn("Uploaded {} but could not delete the local copy", job.fileName(), e);
+        }
+    }
+
+    private static ThreadFactory named(String prefix) {
+        AtomicInteger counter = new AtomicInteger();
+        return runnable -> {
+            Thread thread = new Thread(runnable, prefix + "-" + counter.incrementAndGet());
+            thread.setDaemon(true);
+            return thread;
+        };
+    }
+}

shared/core/src/main/java/dev/photosync/core/upload/UploadEvent.java

@@ -0,0 +1,36 @@
+package dev.photosync.core.upload;
+
+import dev.photosync.core.provider.UploadReceipt;
+
+/**
+ * The transitions worth telling someone about.
+ *
+ * <p>Byte-level progress is deliberately not an event. It changes hundreds of
+ * times per upload, and the only thing that wants it -- the queue screen -- is
+ * redrawing every frame anyway and can just read {@link UploadQueue#snapshot()}.
+ * Emitting it would mean thousands of cross-thread hops for no gain.
+ */
+public sealed interface UploadEvent {
+
+    UploadJob job();
+
+    record Enqueued(UploadJob job) implements UploadEvent {
+    }
+
+    record Started(UploadJob job) implements UploadEvent {
+    }
+
+    record Completed(UploadJob job, UploadReceipt receipt) implements UploadEvent {
+    }
+
+    /** A single attempt failed. {@code willRetry} says whether another one is coming. */
+    record Failed(UploadJob job, String message, boolean willRetry) implements UploadEvent {
+    }
+
+    record Removed(UploadJob job) implements UploadEvent {
+    }
+
+    /** The last active job left the queue. The quit dialog waits for this. */
+    record Drained(UploadJob job) implements UploadEvent {
+    }
+}

shared/core/src/main/java/dev/photosync/core/upload/UploadJob.java

@@ -0,0 +1,62 @@
+package dev.photosync.core.upload;
+
+import dev.photosync.core.capture.CaptureOrigin;
+import dev.photosync.core.provider.AlbumRef;
+import dev.photosync.core.provider.UploadRequest;
+import lombok.Builder;
+
+import java.nio.file.Path;
+import java.time.Instant;
+import java.util.Optional;
+
+/**
+ * One screenshot's journey to the backend, as an immutable value.
+ *
+ * <p>Every transition produces a new job rather than mutating one, so the queue
+ * can hand a consistent snapshot to the GUI while workers are running, without
+ * copying or locking on the render thread.
+ *
+ * <p>The file is held as a string rather than a {@link Path} because this record
+ * is serialized straight to the queue file, and Gson has no idea what to do with
+ * a filesystem-specific {@code Path} implementation.
+ */
+@Builder(toBuilder = true)
+public record UploadJob(
+        String id,
+        String file,
+        String fileName,
+        long sizeBytes,
+        Instant capturedAt,
+        CaptureOrigin origin,
+        String albumId,
+        UploadState state,
+        int attempts,
+        Instant notBefore,
+        String lastError,
+        String assetId) {
+
+    public Path path() {
+        return Path.of(file);
+    }
+
+    public AlbumRef album() {
+        return AlbumRef.of(albumId);
+    }
+
+    public Optional<String> failureMessage() {
+        return Optional.ofNullable(lastError).filter(message -> !message.isBlank());
+    }
+
+    public UploadRequest toRequest() {
+        return new UploadRequest(path(), fileName, capturedAt, capturedAt, album());
+    }
+
+    /** True once the backoff has elapsed and a worker may pick this up. */
+    public boolean isClaimable(Instant now) {
+        return switch (state) {
+            case PENDING -> true;
+            case RETRYING -> notBefore == null || !notBefore.isAfter(now);
+            default -> false;
+        };
+    }
+}

shared/core/src/main/java/dev/photosync/core/upload/UploadQueue.java

@@ -0,0 +1,376 @@
+package dev.photosync.core.upload;
+
+import com.google.gson.reflect.TypeToken;
+import dev.photosync.core.capture.CapturedScreenshot;
+import dev.photosync.core.persistence.JsonFile;
+import dev.photosync.core.provider.AlbumRef;
+import dev.photosync.core.provider.UploadReceipt;
+import lombok.extern.slf4j.Slf4j;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.time.Instant;
+import java.util.ArrayList;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.function.Consumer;
+
+/**
+ * The durable list of screenshots on their way to the backend.
+ *
+ * <p>This is the answer to "what happens if the game dies mid-upload". Every
+ * state change is written through to {@code photosync-queue.json} before it is
+ * announced, so the worst a hard kill can cost is one in-flight request. On the
+ * next launch anything left in {@link UploadState#UPLOADING} is demoted back to
+ * {@link UploadState#PENDING} and simply tried again -- which is safe because
+ * {@code PhotoProvider.upload} is required to answer
+ * {@link UploadReceipt.Outcome#DUPLICATE} rather than store a second copy.
+ *
+ * <p>All mutation happens under one lock and all events are published outside
+ * it, so a listener can call back into the queue without deadlocking.
+ */
+@Slf4j
+public final class UploadQueue {
+
+    private final JsonFile file;
+    private final int historyLimit;
+
+    private final Object lock = new Object();
+    private final Map<String, UploadJob> jobs = new LinkedHashMap<>();
+
+    /** jobId to {sent, total}. Not persisted: meaningless across a restart. */
+    private final Map<String, long[]> transfers = new ConcurrentHashMap<>();
+    private final Set<String> cancelRequests = ConcurrentHashMap.newKeySet();
+    private final List<Consumer<UploadEvent>> listeners = new CopyOnWriteArrayList<>();
+
+    public UploadQueue(Path path) {
+        this(path, 100);
+    }
+
+    public UploadQueue(Path path, int historyLimit) {
+        this.file = new JsonFile(path);
+        this.historyLimit = historyLimit;
+        restore();
+    }
+
+    public void onEvent(Consumer<UploadEvent> listener) {
+        listeners.add(listener);
+    }
+
+    // -----------------------------------------------------------------------
+    // Reading
+    // -----------------------------------------------------------------------
+
+    /** Oldest first, which is also the order workers pick jobs up in. */
+    public List<QueuedUpload> snapshot() {
+        List<UploadJob> current;
+        synchronized (lock) {
+            current = List.copyOf(jobs.values());
+        }
+        List<QueuedUpload> view = new ArrayList<>(current.size());
+        for (UploadJob job : current) {
+            long[] transfer = transfers.get(job.id());
+            view.add(transfer == null
+                    ? QueuedUpload.idle(job)
+                    : new QueuedUpload(job, transfer[0], transfer[1]));
+        }
+        return view;
+    }
+
+    public Optional<UploadJob> find(String id) {
+        synchronized (lock) {
+            return Optional.ofNullable(jobs.get(id));
+        }
+    }
+
+    /** How much work the quit dialog is waiting for. */
+    public int activeCount() {
+        synchronized (lock) {
+            return (int) jobs.values().stream().filter(job -> job.state().isActive()).count();
+        }
+    }
+
+    public int failedCount() {
+        synchronized (lock) {
+            return (int) jobs.values().stream().filter(job -> job.state() == UploadState.FAILED).count();
+        }
+    }
+
+    public boolean hasActiveWork() {
+        synchronized (lock) {
+            return jobs.values().stream().anyMatch(job -> job.state().isActive());
+        }
+    }
+
+    public boolean isCancelRequested(String jobId) {
+        return cancelRequests.contains(jobId);
+    }
+
+    // -----------------------------------------------------------------------
+    // Writing
+    // -----------------------------------------------------------------------
+
+    /**
+     * Adds a screenshot, or returns the job already covering that file.
+     *
+     * <p>The de-duplication matters on the path where a capture is enqueued and
+     * the player immediately quits and relaunches: the restored job and a fresh
+     * rescan would otherwise both try to upload the same file.
+     */
+    public UploadJob enqueue(CapturedScreenshot shot, AlbumRef album) {
+        String absolute = shot.file().toAbsolutePath().toString();
+        UploadJob created;
+        synchronized (lock) {
+            Optional<UploadJob> existing = jobs.values().stream()
+                    .filter(job -> job.file().equals(absolute))
+                    .filter(job -> job.state().isActive() || job.state() == UploadState.FAILED)
+                    .findFirst();
+            if (existing.isPresent()) {
+                return existing.get();
+            }
+            created = UploadJob.builder()
+                    .id(UUID.randomUUID().toString())
+                    .file(absolute)
+                    .fileName(shot.fileName())
+                    .sizeBytes(shot.sizeBytes())
+                    .capturedAt(shot.capturedAt())
+                    .origin(shot.origin())
+                    .albumId(album.id().orElse(""))
+                    .state(UploadState.PENDING)
+                    .attempts(0)
+                    .build();
+            jobs.put(created.id(), created);
+            persist();
+        }
+        publish(new UploadEvent.Enqueued(created));
+        return created;
+    }
+
+    /** Takes the oldest job that is due, marking it in flight. */
+    public Optional<UploadJob> claim(Instant now) {
+        UploadJob claimed = null;
+        synchronized (lock) {
+            for (UploadJob job : jobs.values()) {
+                if (job.isClaimable(now)) {
+                    claimed = job.toBuilder()
+                            .state(UploadState.UPLOADING)
+                            .attempts(job.attempts() + 1)
+                            .lastError(null)
+                            .build();
+                    jobs.put(claimed.id(), claimed);
+                    persist();
+                    break;
+                }
+            }
+        }
+        if (claimed != null) {
+            transfers.put(claimed.id(), new long[]{0L, claimed.sizeBytes()});
+            publish(new UploadEvent.Started(claimed));
+        }
+        return Optional.ofNullable(claimed);
+    }
+
+    public void reportProgress(String jobId, long sent, long total) {
+        transfers.put(jobId, new long[]{sent, total});
+    }
+
+    public void succeed(String jobId, UploadReceipt receipt) {
+        transition(jobId, job -> job.toBuilder()
+                .state(UploadState.COMPLETED)
+                .assetId(receipt.assetId())
+                .lastError(null)
+                .build(),
+                job -> new UploadEvent.Completed(job, receipt));
+    }
+
+    /** A retryable failure: the job goes back to sleep until {@code notBefore}. */
+    public void reschedule(String jobId, String message, Instant notBefore) {
+        transition(jobId, job -> job.toBuilder()
+                .state(UploadState.RETRYING)
+                .notBefore(notBefore)
+                .lastError(message)
+                .build(),
+                job -> new UploadEvent.Failed(job, message, true));
+    }
+
+    /** A permanent failure, or the last attempt. Waits for the player now. */
+    public void fail(String jobId, String message) {
+        transition(jobId, job -> job.toBuilder()
+                .state(UploadState.FAILED)
+                .notBefore(null)
+                .lastError(message)
+                .build(),
+                job -> new UploadEvent.Failed(job, message, false));
+    }
+
+    /** Player pressed retry: a clean slate rather than one more attempt. */
+    public void retry(String jobId) {
+        transition(jobId, job -> job.toBuilder()
+                .state(UploadState.PENDING)
+                .attempts(0)
+                .notBefore(null)
+                .lastError(null)
+                .build(),
+                UploadEvent.Enqueued::new);
+    }
+
+    public void retryAllFailed() {
+        List<String> failed;
+        synchronized (lock) {
+            failed = jobs.values().stream()
+                    .filter(job -> job.state() == UploadState.FAILED)
+                    .map(UploadJob::id)
+                    .toList();
+        }
+        failed.forEach(this::retry);
+    }
+
+    /**
+     * Removes a job. An upload already in flight is asked to stop through
+     * {@link #isCancelRequested(String)}, which its progress callback checks.
+     */
+    public void cancel(String jobId) {
+        cancelRequests.add(jobId);
+        transition(jobId, job -> job.toBuilder()
+                .state(UploadState.CANCELLED)
+                .lastError(null)
+                .build(),
+                UploadEvent.Removed::new);
+    }
+
+    /** Drops a finished or failed job from the list entirely. */
+    public void forget(String jobId) {
+        UploadJob removed;
+        synchronized (lock) {
+            UploadJob job = jobs.get(jobId);
+            if (job == null || job.state() == UploadState.UPLOADING) {
+                return;
+            }
+            removed = jobs.remove(jobId);
+            persist();
+        }
+        transfers.remove(jobId);
+        cancelRequests.remove(jobId);
+        publish(new UploadEvent.Removed(removed));
+    }
+
+    public void clearFinished() {
+        List<UploadJob> removed = new ArrayList<>();
+        synchronized (lock) {
+            jobs.values().removeIf(job -> {
+                if (job.state().isFinished()) {
+                    removed.add(job);
+                    return true;
+                }
+                return false;
+            });
+            if (!removed.isEmpty()) {
+                persist();
+            }
+        }
+        for (UploadJob job : removed) {
+            transfers.remove(job.id());
+            cancelRequests.remove(job.id());
+            publish(new UploadEvent.Removed(job));
+        }
+    }
+
+    // -----------------------------------------------------------------------
+    // Internals
+    // -----------------------------------------------------------------------
+
+    private void transition(String jobId,
+                            java.util.function.UnaryOperator<UploadJob> change,
+                            java.util.function.Function<UploadJob, UploadEvent> event) {
+        UploadJob updated;
+        boolean drained;
+        synchronized (lock) {
+            UploadJob previous = jobs.get(jobId);
+            if (previous == null) {
+                return;
+            }
+            updated = change.apply(previous);
+            jobs.put(jobId, updated);
+            trimHistory();
+            persist();
+            drained = previous.state().isActive() && jobs.values().stream().noneMatch(job -> job.state().isActive());
+        }
+        if (!updated.state().isActive()) {
+            transfers.remove(jobId);
+        }
+        publish(event.apply(updated));
+        if (drained) {
+            publish(new UploadEvent.Drained(updated));
+        }
+    }
+
+    /** Caller holds the lock. Keeps the finished tail from growing without bound. */
+    private void trimHistory() {
+        List<String> finished = jobs.values().stream()
+                .filter(job -> job.state().isFinished())
+                .map(UploadJob::id)
+                .toList();
+        for (int i = 0; i < finished.size() - historyLimit; i++) {
+            jobs.remove(finished.get(i));
+        }
+    }
+
+    /** Caller holds the lock. */
+    private void persist() {
+        List<UploadJob> durable = jobs.values().stream()
+                .filter(job -> job.state().isPersistent())
+                .toList();
+        try {
+            file.write(file.gson().toJsonTree(durable));
+        } catch (IOException e) {
+            log.error("Could not write the upload queue to {}", file.path(), e);
+        }
+    }
+
+    private void restore() {
+        TypeToken<List<UploadJob>> listType = new TypeToken<>() {
+        };
+        List<UploadJob> stored = file.readTree()
+                .<List<UploadJob>>map(tree -> file.gson().fromJson(tree, listType.getType()))
+                .orElseGet(List::of);
+
+        int skipped = 0;
+        for (UploadJob job : stored) {
+            if (job == null || job.id() == null || job.file() == null || job.state() == null) {
+                skipped++;
+                continue;
+            }
+            if (!Files.isRegularFile(Path.of(job.file()))) {
+                // The screenshot was deleted while we were away. Nothing to send.
+                skipped++;
+                continue;
+            }
+            // Anything that claimed to be uploading was interrupted by whatever
+            // ended the last session; start it over.
+            UploadJob resumed = job.state() == UploadState.UPLOADING
+                    ? job.toBuilder().state(UploadState.PENDING).build()
+                    : job;
+            jobs.put(resumed.id(), resumed);
+        }
+        if (!jobs.isEmpty() || skipped > 0) {
+            log.info("Restored {} pending upload(s) from {} ({} dropped)", jobs.size(), file.path(), skipped);
+        }
+    }
+
+    private void publish(UploadEvent event) {
+        for (Consumer<UploadEvent> listener : listeners) {
+            try {
+                listener.accept(event);
+            } catch (RuntimeException e) {
+                log.error("An upload listener failed on {}", event.getClass().getSimpleName(), e);
+            }
+        }
+    }
+}

shared/core/src/main/java/dev/photosync/core/upload/UploadState.java

@@ -0,0 +1,40 @@
+package dev.photosync.core.upload;
+
+/**
+ * Where a job is in its life.
+ *
+ * <p>{@link #FAILED} is neither active nor finished on purpose: the job has run
+ * out of automatic attempts and is now waiting for the player to press retry.
+ * Treating it as finished would quietly lose screenshots; treating it as active
+ * would keep the quit dialog open forever.
+ */
+public enum UploadState {
+
+    /** Waiting for a free worker. */
+    PENDING,
+    /** A worker is pushing bytes right now. */
+    UPLOADING,
+    /** A retryable error; will be picked up again after a backoff. */
+    RETRYING,
+    /** The backend has the file. */
+    COMPLETED,
+    /** Out of attempts, or a permanent error. Needs the player. */
+    FAILED,
+    /** The player removed it before it went up. */
+    CANCELLED;
+
+    /** Counts as outstanding work: the quit dialog waits for these. */
+    public boolean isActive() {
+        return this == PENDING || this == UPLOADING || this == RETRYING;
+    }
+
+    /** Nothing more will happen without the player asking. */
+    public boolean isFinished() {
+        return this == COMPLETED || this == CANCELLED;
+    }
+
+    /** Worth writing to the queue file, because it still means something next launch. */
+    public boolean isPersistent() {
+        return isActive() || this == FAILED;
+    }
+}

shared/core/src/test/java/dev/photosync/core/thumbnail/ThumbHashTest.java

@@ -0,0 +1,132 @@
+package dev.photosync.core.thumbnail;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import java.util.Optional;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * The decoder is a transcription of a bit-packed format, so these tests aim at
+ * the things a transcription gets wrong: which bits a field lives in, how the
+ * shared nibble stream is walked, and the colour-space conversion at the end.
+ *
+ * <p>Hashes are built here rather than captured from a server, because setting
+ * the three AC scales to zero collapses the whole DCT to its DC term -- giving a
+ * flat image whose exact colour can be worked out by hand.
+ */
+class ThumbHashTest {
+
+    @Test
+    @DisplayName("a hash with no AC energy decodes to its DC colour, everywhere")
+    void flatImage() {
+        // l_dc = 42/63, p_dc = 63 -> +1.0, q_dc = 10 -> -0.68254, l_scale = 0.
+        // blue  = 0.66667 - 2/3 * 1.0            = 0.0
+        // red   = (3*0.66667 - 0.0 + -0.68254)/2 = 0.65873 -> 168
+        // green = red - q                        = 1.34127 -> 255 (clamped)
+        byte[] hash = hash(42, 63, 10, 0, 7, 0, 0);
+
+        ThumbImage image = ThumbHash.decode(hash).orElseThrow();
+
+        assertEquals(32, image.width());
+        assertEquals(32, image.height());
+        for (int y = 0; y < image.height(); y++) {
+            for (int x = 0; x < image.width(); x++) {
+                int pixel = image.pixel(x, y);
+                assertEquals(255, alpha(pixel), "opaque when the hash carries no alpha");
+                assertNear(168, red(pixel), "red");
+                assertNear(255, green(pixel), "green");
+                assertNear(0, blue(pixel), "blue");
+            }
+        }
+    }
+
+    @Test
+    @DisplayName("a landscape hash is decoded at its own aspect ratio")
+    void aspectRatio() {
+        // Landscape with no alpha pins lx to 7 and reads ly from the low nibble:
+        // 7/3 -> 32 x round(32 / 2.3333) = 32 x 14.
+        ThumbImage image = ThumbHash.decode(landscape(3)).orElseThrow();
+
+        assertEquals(32, image.width());
+        assertEquals(14, image.height());
+    }
+
+    @Test
+    @DisplayName("AC coefficients actually reach the pixels")
+    void acCoefficientsVaryTheImage() {
+        // Same DC as the flat case but with full luminance scale, so the AC
+        // nibbles -- which are 0x5A filler -- must show up as variation.
+        ThumbImage image = ThumbHash.decode(hash(42, 63, 10, 31, 7, 63, 63)).orElseThrow();
+
+        int first = image.pixel(0, 0);
+        boolean varies = false;
+        for (int y = 0; y < image.height() && !varies; y++) {
+            for (int x = 0; x < image.width(); x++) {
+                if (image.pixel(x, y) != first) {
+                    varies = true;
+                    break;
+                }
+            }
+        }
+        assertTrue(varies, "a non-zero l_scale must produce a non-flat image");
+        assertNotEquals(0, image.argb().length);
+    }
+
+    @Test
+    @DisplayName("malformed input yields no image rather than an exception")
+    void rejectsGarbage() {
+        assertEquals(Optional.empty(), ThumbHash.decode((String) null));
+        assertEquals(Optional.empty(), ThumbHash.decode(""));
+        assertEquals(Optional.empty(), ThumbHash.decode("not base64 at all!!"));
+        assertEquals(Optional.empty(), ThumbHash.decode(new byte[] {1, 2, 3}));
+        // Well-formed header, but the AC nibble stream is truncated.
+        assertEquals(Optional.empty(), ThumbHash.decode(new byte[] {(byte) 0xEA, (byte) 0xAF, 0, 7, 0, 0x5A}));
+    }
+
+    /** Builds a portrait, alpha-less hash from the raw field values. */
+    private static byte[] hash(int lDc, int pDc, int qDc, int lScale, int lCount, int pScale, int qScale) {
+        int header24 = (lDc & 63) | ((pDc & 63) << 6) | ((qDc & 63) << 12) | ((lScale & 31) << 18);
+        int header16 = (lCount & 7) | ((pScale & 63) << 3) | ((qScale & 63) << 9);
+        byte[] hash = new byte[24];
+        hash[0] = (byte) header24;
+        hash[1] = (byte) (header24 >> 8);
+        hash[2] = (byte) (header24 >> 16);
+        hash[3] = (byte) header16;
+        hash[4] = (byte) (header16 >> 8);
+        for (int i = 5; i < hash.length; i++) {
+            hash[i] = 0x5A;
+        }
+        return hash;
+    }
+
+    private static byte[] landscape(int lyCount) {
+        byte[] hash = hash(42, 63, 10, 0, lyCount, 0, 0);
+        hash[4] |= (byte) 0x80;
+        return hash;
+    }
+
+    private static void assertNear(int expected, int actual, String channel) {
+        assertTrue(Math.abs(expected - actual) <= 2,
+                () -> channel + " expected around " + expected + " but was " + actual);
+    }
+
+    private static int alpha(int argb) {
+        return (argb >>> 24) & 0xFF;
+    }
+
+    private static int red(int argb) {
+        return (argb >>> 16) & 0xFF;
+    }
+
+    private static int green(int argb) {
+        return (argb >>> 8) & 0xFF;
+    }
+
+    private static int blue(int argb) {
+        return argb & 0xFF;
+    }
+}