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/stores/nativeVideo.ts

@@ -26,13 +26,27 @@ const STORAGE_KEY = "jellytau-experimental-native-video";
 /** The attribute app.css keys its transparency rules off. */
 const NATIVE_VIDEO_ATTR = "data-native-video";
 
+/**
+ * Whether the native path is on, defaulting to **on** when the user has never
+ * chosen.
+ *
+ * It shipped defaulting to off while the native path was a spike. It is now the
+ * default because picture-in-picture is built on it: PiP shrinks the *Activity*,
+ * so it needs a real video surface behind the WebView to show, and on the HTML5
+ * path there is nothing for it to shrink into but the UI itself (DR-160).
+ *
+ * An explicit stored choice still wins in both directions, so anyone who turned
+ * it off keeps it off.
+ */
 function load(): boolean {
-  if (typeof localStorage === "undefined") return false;
+  if (typeof localStorage === "undefined") return true;
   try {
-    return localStorage.getItem(STORAGE_KEY) === "true";
+    const stored = localStorage.getItem(STORAGE_KEY);
+    return stored === null ? true : stored === "true";
   } catch {
-    // Private-mode / disabled storage — default to the safe (HTML5) path.
-    return false;
+    // Private-mode / disabled storage — no stored choice is readable, so this is
+    // the same case as "never chosen".
+    return true;
   }
 }