Add: support for user-global configuration.

platform/common/src/main/java/dev/photosync/platform/UserDataDirectory.java

@@ -0,0 +1,25 @@
+package dev.photosync.platform;
+
+import java.nio.file.Path;
+
+/**
+ * The OS-user-level directory PhotoSync offers for its settings file.
+ *
+ * <p>One rule, written once: Windows keeps per-user application data under
+ * {@code %APPDATA%}, everything else under {@code ~/.config} (the convention
+ * the Linux desktops and macOS share). Nine buckets compile this class, so a
+ * fix to the rule needs one edit instead of nine.
+ */
+public final class UserDataDirectory {
+
+    private UserDataDirectory() {
+    }
+
+    public static Path resolve() {
+        String appData = System.getenv("APPDATA");
+        Path base = appData == null || appData.isBlank()
+                ? Path.of(System.getProperty("user.home"), ".config")
+                : Path.of(appData);
+        return base.resolve("photosyncmod");
+    }
+}