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/+layout.svelte

@@ -3,6 +3,7 @@
   import { goto } from "$app/navigation";
   import { isAuthenticated, isLoading as isAuthLoading } from "$lib/stores/auth";
   import { useScrollGuard } from "$lib/composables/useScrollGuard";
+  import { useScrollRestore } from "$lib/utils/scrollContainer";
   import AppHeader from "$lib/components/AppHeader.svelte";
   import BottomUi from "$lib/components/BottomUi.svelte";
   import SleepTimerModal from "$lib/components/player/SleepTimerModal.svelte";
@@ -11,6 +12,12 @@
   const scrollGuard = useScrollGuard(300);
   setContext("scrollGuard", scrollGuard);
 
+  // This scroller outlives every /library/* route rendered into it, so opening
+  // an item from half-way down a grid used to drop the viewer half-way down the
+  // detail page. Registered at init, as SvelteKit's nav hooks require. (DR-156)
+  let libraryScroller = $state<HTMLElement>();
+  useScrollRestore(() => libraryScroller, "library");
+
   let { children } = $props();
 
   let showSleepTimerModal = $state(false);
@@ -45,6 +52,7 @@
          scroller is physically bounded above it and its last row can never
          render behind the nav — no measurement, no reserved padding. -->
     <main
+      bind:this={libraryScroller}
       class="flex-1 overflow-y-auto p-4 min-h-0"
       style="overscroll-behavior: contain"
       onscroll={scrollGuard.onScroll}