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/scrollRestore.ts

@@ -0,0 +1,91 @@
+/**
+ * Per-route scroll memory for the app's persistent scroll containers.
+ *
+ * The shell keeps its scrollers alive across navigation on purpose: the root
+ * layout, the home page and the library layout each own a
+ * `flex-1 overflow-y-auto` box that outlives the route rendered inside it. That
+ * is what makes the bottom UI a flex sibling rather than a measured overlay —
+ * but it also means the *element* never remounts, so its `scrollTop` survives a
+ * route change and the next page opens part-way down.
+ *
+ * SvelteKit's own scroll restoration cannot help here: it saves and restores
+ * `window` scroll, and in this app the window never scrolls at all.
+ *
+ * So each container gets its own memory, which reproduces normal browser
+ * behaviour:
+ *
+ *  - **forward** (link/goto/form) — a fresh visit, always lands at the top;
+ *  - **popstate** (hardware/gesture Back or Forward) — restores the offset the
+ *    route was left at, so Back out of a detail page returns you to your place
+ *    in the list rather than to the top of it;
+ *  - **enter** (initial load) — left alone; there is nothing to leak yet.
+ *
+ * The decision is pure and lives here so it can be unit-tested without a DOM;
+ * `scrollContainer.svelte.ts` is the thin action that applies it.
+ *
+ * TRACES: UR-054 | DR-156
+ */
+
+/** How a navigation should affect a persistent scroll container. */
+export type NavKind = "enter" | "popstate" | "forward";
+
+/** What to do with the container once the new route has rendered. */
+export type ScrollAction =
+  | { kind: "reset" }
+  | { kind: "restore"; top: number }
+  | { kind: "none" };
+
+/**
+ * Collapse SvelteKit's navigation types into the three cases that matter.
+ *
+ * `enter` is the initial load. `popstate` is a Back/Forward gesture. Everything
+ * else — `link`, `goto`, `form` — is a forward move into a new page.
+ */
+export function classifyNavigation(nav: { type?: string | null }): NavKind {
+  if (nav.type === "enter") return "enter";
+  if (nav.type === "popstate") return "popstate";
+  return "forward";
+}
+
+/**
+ * Remembers the offset each route was left at, for one scroll container.
+ *
+ * One instance per container: the root scroller, the home scroller and the
+ * library scroller hold different content for the same URL, so a shared map
+ * would restore one container's offset into another.
+ */
+export class ScrollMemory {
+  #offsets = new Map<string, number>();
+
+  /** Record where `key` was scrolled to, before we navigate away from it. */
+  save(key: string, top: number): void {
+    this.#offsets.set(key, Math.max(0, top));
+  }
+
+  /**
+   * Decide what the container should do on arriving at `key`.
+   *
+   * Note this does not consume the saved offset: a route can be returned to
+   * more than once, and each Back should restore the same place.
+   */
+  decide(key: string, kind: NavKind): ScrollAction {
+    if (kind === "enter") return { kind: "none" };
+    if (kind === "popstate") return { kind: "restore", top: this.#offsets.get(key) ?? 0 };
+    return { kind: "reset" };
+  }
+
+  /** Drop everything. Intended for tests and sign-out. */
+  clear(): void {
+    this.#offsets.clear();
+  }
+}
+
+/**
+ * The memory key for a URL.
+ *
+ * Path plus query: a library grid filtered by genre is a different list from
+ * the unfiltered one, and returning to it should restore its own place.
+ */
+export function scrollKey(url: { pathname: string; search?: string }): string {
+  return `${url.pathname}${url.search ?? ""}`;
+}