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/routes/library/[id]/+page.svelte

@@ -19,6 +19,7 @@
   import EpisodeFocusView from "$lib/components/library/EpisodeFocusView.svelte";
   import SeriesDownloadButton from "$lib/components/library/SeriesDownloadButton.svelte";
   import ClearHistoryButton from "$lib/components/library/ClearHistoryButton.svelte";
+  import WatchedToggleButton from "$lib/components/library/WatchedToggleButton.svelte";
   import VideoDownloadButton from "$lib/components/library/VideoDownloadButton.svelte";
   import FavoriteButton from "$lib/components/FavoriteButton.svelte";
   import { favoriteOverrides, resolveIsFavorite } from "$lib/stores/favorites";
@@ -549,6 +550,14 @@
                 seriesName={item.name}
                 episodeCount={allEpisodes.length || undefined}
               />
+              <WatchedToggleButton
+                itemId={item.id}
+                watched={allEpisodes.length > 0 &&
+                  allEpisodes.every((e) => e.userData?.isPlayed)}
+                scope="series"
+                showLabel={true}
+                onChanged={loadItem}
+              />
               <ClearHistoryButton
                 itemId={item.id}
                 itemName={item.name}
@@ -562,6 +571,14 @@
                 isMovie={true}
                 size="lg"
               />
+              <!-- A movie is a leaf, so its own played flag is the whole story. -->
+              <WatchedToggleButton
+                itemId={item.id}
+                watched={item.userData?.isPlayed ?? false}
+                scope="episode"
+                showLabel={true}
+                onChanged={loadItem}
+              />
             {/if}
             <!-- Favourite. Sits with Play/Download rather than in the header,
                  per ux-flows §5B.3/§5B.4. TRACES: UR-068 | DR-119 -->