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/api/bindings.ts

@@ -746,6 +746,33 @@ async storageUpdatePlaybackContext(userId: string, itemId: string, positionMs: n
 async storageMarkPlayed(userId: string, itemId: string) : Promise<null> {
     return await TAURI_INVOKE("storage_mark_played", { userId, itemId });
 },
+/**
+ * Set the watched flag locally for an item **and everything inside it**.
+ * 
+ * This backs the watched toggle, and is deliberately separate from
+ * [`storage_mark_played`] — which reports a single track/episode finishing and
+ * increments `play_count` — because the toggle has two directions and applies
+ * to containers.
+ * 
+ * The recursion is what makes the toggle honest offline. Jellyfin applies
+ * `POST`/`DELETE /PlayedItems/{id}` recursively over a season or series, so
+ * online the server fixes up the children on the next read; with no server to
+ * ask, marking a season watched would otherwise tick the season and leave every
+ * episode inside it unwatched. Targets are drawn from `items` by the same link
+ * columns the rest of the offline layer uses, so an id that is not cached
+ * selects nothing and the statement is a no-op rather than a foreign-key error.
+ * 
+ * Un-marking clears the resume position too, matching the server, so an item
+ * un-marked offline does not come back offering to resume from a position it is
+ * no longer meant to have.
+ * 
+ * `pending_sync = 1` hands the rows to the sync drain.
+ * 
+ * TRACES: UR-073 | DR-158
+ */
+async storageSetWatched(userId: string, itemId: string, watched: boolean) : Promise<null> {
+    return await TAURI_INVOKE("storage_set_watched", { userId, itemId, watched });
+},
 /**
  * Get playback progress for an item
  */