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/utils/scrollContainer.ts

@@ -0,0 +1,90 @@
+/**
+ * Wires a persistent scroll container to the per-route scroll memory.
+ *
+ * The decision logic is pure and lives in `scrollRestore.ts`; this is the thin
+ * DOM/SvelteKit half. Call it once at component init (SvelteKit's navigation
+ * hooks must be registered during initialisation, not from `onMount`), passing
+ * a getter for the element — the element itself is bound later, so a getter is
+ * the only way to hand it over from the top of `<script>`.
+ *
+ *   let scroller: HTMLElement | undefined = $state();
+ *   useScrollRestore(() => scroller, "library");
+ *   …
+ *   <div bind:this={scroller} class="flex-1 overflow-y-auto">
+ *
+ * Memories are keyed by container id and held at module scope, not per call.
+ * Two containers must never share one (the root, home and library scrollers
+ * hold different content for the same URL, so a shared map would restore one
+ * into another) — but a container that *remounts* has to find its offsets again
+ * when it comes back. The home scroller is destroyed on every navigation away,
+ * so a memory owned by the component instance would be empty on return and Back
+ * could only ever land at the top.
+ *
+ * TRACES: UR-072 | DR-156
+ */
+
+import { beforeNavigate, afterNavigate } from "$app/navigation";
+import { tick } from "svelte";
+import { ScrollMemory, classifyNavigation, scrollKey } from "./scrollRestore";
+
+/** Container id → its offsets. Outlives the components that mount them. */
+const memories = new Map<string, ScrollMemory>();
+
+function memoryFor(containerId: string): ScrollMemory {
+  let memory = memories.get(containerId);
+  if (!memory) {
+    memory = new ScrollMemory();
+    memories.set(containerId, memory);
+  }
+  return memory;
+}
+
+/** Forget every container's offsets. For sign-out and tests. */
+export function clearScrollMemories(): void {
+  memories.clear();
+}
+
+export function useScrollRestore(
+  getElement: () => HTMLElement | null | undefined,
+  containerId: string
+): void {
+  const memory = memoryFor(containerId);
+
+  // Record where we were before the route changes. `nav.from` is absent on the
+  // very first navigation, which is exactly when there is nothing to save.
+  beforeNavigate((nav) => {
+    const element = getElement();
+    if (!element || !nav.from) return;
+    memory.save(scrollKey(nav.from.url), element.scrollTop);
+  });
+
+  afterNavigate(async (nav) => {
+    const target = nav.to;
+    if (!target) return;
+
+    const action = memory.decide(scrollKey(target.url), classifyNavigation(nav));
+    if (action.kind === "none") return;
+
+    const top = action.kind === "restore" ? action.top : 0;
+
+    // Wait for the new route's markup to be in the DOM before moving the
+    // scroller — setting scrollTop past the current content height is clamped,
+    // and a reset applied too early is undone by the incoming render.
+    await tick();
+    const element = getElement();
+    if (!element) return;
+
+    element.scrollTop = top;
+
+    // A restore often targets content that is still loading (a library grid
+    // fetches after mount), so the offset would clamp to a short page. Re-apply
+    // on the next frame, once, which is enough for the common case without
+    // fighting a user who has already started scrolling.
+    if (action.kind === "restore" && top > 0) {
+      requestAnimationFrame(() => {
+        const el = getElement();
+        if (el && el.scrollTop < top) el.scrollTop = top;
+      });
+    }
+  });
+}