Add: support for user-global configuration.

shared/mc-api/src/main/java/dev/photosync/mcapi/GameContext.java

@@ -26,9 +26,23 @@ public interface GameContext {
      */
     Optional<CameraPose> cameraPose();
 
-    /** {@code .minecraft/config}, where PhotoSync keeps its settings and upload queue. */
+    /**
+     * The game-instance config directory (the {@code config/photosync} folder
+     * inside the game directory), where PhotoSync historically keeps its
+     * settings and upload queue. The settings file can be moved to
+     * {@link #userDataDirectory()} from the settings screen; the upload queue
+     * always stays here.
+     */
     Path configDirectory();
 
+    /**
+     * The OS-user-level directory offered as the alternative home for the
+     * settings file: {@code ~/.config/photosyncmod} on Linux and macOS,
+     * {@code %APPDATA%\photosyncmod} on Windows. Only used once the player
+     * chooses it; {@link #configDirectory()} stays the default.
+     */
+    Path userDataDirectory();
+
     /**
      * Runs a task on the render thread, or immediately if already on it.
      *