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/immersive.ts

@@ -0,0 +1,68 @@
+/**
+ * Immersive (system-bar-free) full-screen video, Android only.
+ *
+ * TRACES: UR-066 | DR-157
+ *
+ * `requestFullscreen()` is the only fullscreen control the web layer has, and in
+ * an Android WebView it does not touch the Activity window — it expands the
+ * element inside a viewport that already spans the whole screen (MainActivity
+ * calls `enableEdgeToEdge()`, and SDK 36 makes that mandatory). So the status and
+ * navigation bars stayed painted over full-screen video, and "fullscreen"
+ * changed nothing visible.
+ *
+ * Hiding them needs `WindowInsetsControllerCompat` on the Activity, so it goes
+ * through the `AndroidImmersive` @JavascriptInterface installed by MainActivity.
+ * Elsewhere (desktop, the Linux WebKitGTK webview) the real `requestFullscreen()`
+ * already does the right thing and these calls are no-ops.
+ */
+
+interface AndroidImmersiveBridge {
+  enter(): void;
+  exit(): void;
+  isSupported(): boolean;
+}
+
+declare global {
+  interface Window {
+    AndroidImmersive?: AndroidImmersiveBridge;
+  }
+}
+
+function bridge(): AndroidImmersiveBridge | undefined {
+  if (typeof window === "undefined") return undefined;
+  return window.AndroidImmersive;
+}
+
+/** Whether native immersive mode exists on this platform. */
+export function isImmersiveSupported(): boolean {
+  try {
+    return bridge()?.isSupported() ?? false;
+  } catch (err) {
+    console.warn("[Immersive] isSupported check failed:", err);
+    return false;
+  }
+}
+
+/** Hide the system bars. No-op where unsupported. */
+export function enterImmersive(): void {
+  try {
+    bridge()?.enter();
+  } catch (err) {
+    console.error("[Immersive] Failed to hide the system bars:", err);
+  }
+}
+
+/**
+ * Restore the system bars. No-op where unsupported.
+ *
+ * Call this on leaving fullscreen *and* on player teardown — the bars belong to
+ * the Activity, not the player, so a player destroyed while immersive would
+ * leave every screen behind it without a status or navigation bar.
+ */
+export function exitImmersive(): void {
+  try {
+    bridge()?.exit();
+  } catch (err) {
+    console.error("[Immersive] Failed to restore the system bars:", err);
+  }
+}