fix(ui,player): scroll restore, immersive fullscreen, watched toggle, handoff timeline, PiP

Batch of reported bugs and enhancements.

UI
- Pages no longer inherit the previous page's scroll position (DR-156, UR-072).
  The shell keeps its scrollers alive across navigation by design, so the
  element never remounts and its scrollTop survived the route change; SvelteKit
  restores window scroll, which this app never uses. ScrollMemory records the
  offset per route and per container: forward moves reset to the top, Back
  restores where the route was left.
- Season header stacks on narrow screens, and the title span gets min-w-0 so it
  actually truncates instead of overflowing under the action buttons.
- Favourites gets a labelled tile at the head of the library grid rather than
  only an unlabelled heart icon in the header.

Playback
- Full-screen video on Android hides the system bars (DR-157, UR-066).
  requestFullscreen() cannot touch the Activity window from inside a WebView, so
  the control did nothing visible while the bars stayed painted over the video.
  ImmersiveModeBridge hides them, restored on exit, Escape and teardown.
- Background-audio handoff stops leaking its relative timeline (DR-159).
  background_audio_base was a display-only correction applied in two places
  while progress reports to Jellyfin, the frontend and media3's own seeks all
  worked in the relative timeline treating it as absolute — each crossing losing
  exactly `base` seconds. The conversion now happens once, in the position tick,
  and inbound seeks resolve through seek_absolute, which re-opens the stream at
  the requested position because the handoff transcode cannot seek.
- Picture-in-picture works on the path that actually plays video (DR-160).
  canEnterPip demanded a native ExoPlayer surface, but that path is behind a
  flag defaulting to off, so PiP could never engage. It now accepts the WebView
  <video> too, keeping the WebView visible and routing play/pause to the element.
- Native video is now the default so PiP has a real surface (DR-161). The
  scrub-regression tests pinned the flag-off path implicitly; they now mock it
  off explicitly. The native scrub/seek path is not covered by the suite and
  needs device verification.

Watched state
- Watched toggle on the episode row, season header, series and movie hero, and
  the Episode Focus View (DR-158, UR-073). Both backend halves already existed
  with no caller. storage_set_watched covers a container's episodes so the
  toggle is honest offline, and QueuedOp::MarkUnplayed gives the sync queue the
  missing direction.

Release
- Fix the Android versionCode floor (set-version.sh). v0.5.2 shipped code 5002
  under an earlier minor*1000 scheme, but the current minor*100 formula yields
  1502 for that version and 1503 for 0.5.3 — so every 0.5.x release built from
  it was an un-installable downgrade for anyone already on v0.5.2. Widened to
  10000 + major*1000000 + minor*1000 + patch (0.5.3 -> 15003).
- Bump to 0.5.3.

src/lib/services/syncService.ts

@@ -17,6 +17,7 @@ export type { SyncQueueItem };
 
 export type SyncOperation =
   | "mark_played"
+  | "mark_unplayed"
   | "mark_favorite"
   | "unmark_favorite"
   | "update_progress"
@@ -101,12 +102,30 @@ class SyncService {
    * Also updates local state immediately
    */
   async queueMarkPlayed(itemId: string): Promise<number> {
-    // Update local state first
-    await commands.storageMarkPlayed(auth.getUserId() ?? "", itemId);
+    // storageSetWatched, not storageMarkPlayed: this is the watched *toggle*, so
+    // it has to cover a season or series' episodes too. storageMarkPlayed stays
+    // the single-item "this finished playing" path.
+    await commands.storageSetWatched(auth.getUserId() ?? "", itemId, true);
 
     return this.queueMutation("mark_played", itemId);
   }
 
+  /**
+   * Queue mark as unwatched, the inverse of {@link queueMarkPlayed}.
+   *
+   * Same shape deliberately: the watched toggle has to work in both directions
+   * offline, or un-marking would be the one half that needs a connection. The
+   * drain pushes this as `clear_watch_history` — Jellyfin's mark-unplayed, which
+   * is recursive over a season or series and also clears resume positions.
+   *
+   * TRACES: UR-073 | DR-158
+   */
+  async queueMarkUnplayed(itemId: string): Promise<number> {
+    await commands.storageSetWatched(auth.getUserId() ?? "", itemId, false);
+
+    return this.queueMutation("mark_unplayed", itemId);
+  }
+
   /**
    * Get count of pending sync operations
    */