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/+page.svelte

@@ -10,8 +10,15 @@
   import HeroBanner from "$lib/components/home/HeroBanner.svelte";
   import Carousel from "$lib/components/home/Carousel.svelte";
   import MediaCard from "$lib/components/library/MediaCard.svelte";
+  import { useScrollRestore } from "$lib/utils/scrollContainer";
   import type { MediaItem, Library } from "$lib/api/types";
 
+  // Home scrolls in its own box rather than the shell's, and is destroyed on
+  // every navigation away — so its offsets live in the module-level memory,
+  // letting Back return the viewer to their row instead of the top. (DR-156)
+  let homeScroller = $state<HTMLElement>();
+  useScrollRestore(() => homeScroller, "home");
+
   // Track if we've done an initial load (plain variable, not reactive)
   let hasLoadedOnce = false;
   let previousServerReachable = false;
@@ -147,7 +154,7 @@
     <div class="w-8 h-8 border-2 border-[var(--color-jellyfin)] border-t-transparent rounded-full animate-spin"></div>
   </div>
 {:else}
-  <div class="h-full overflow-y-auto p-4 pb-16 md:pb-4 {isAndroid && $currentMedia && $currentMedia.type !== 'Movie' && $currentMedia.type !== 'Episode' ? 'pb-40' : ''}">
+  <div bind:this={homeScroller} class="h-full overflow-y-auto p-4 pb-16 md:pb-4 {isAndroid && $currentMedia && $currentMedia.type !== 'Movie' && $currentMedia.type !== 'Episode' ? 'pb-40' : ''}">
     <div class="space-y-8">
 
     <!-- Hero Banner -->