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.

scripts/set-version.test.ts

@@ -106,21 +106,33 @@ describe("set-version.sh", () => {
   });
 
   describe("Android versionCode", () => {
-    // Codes below 1000 are already in the field; a newer release must never
-    // produce a smaller number than an older one.
-    it("clears the 1000 floor shipped by earlier builds", () => {
+    // A newer release must never produce a smaller number than an older one, or
+    // Android refuses the update. The floor tracks the highest code actually in
+    // the field, which is NOT the same as the highest this formula has produced:
+    // v0.5.2 shipped versionCode 5002 from an earlier `minor*1000` scheme, while
+    // the `minor*100` formula that replaced it yields only 1502 for that same
+    // version — so every 0.5.x release built from it was an un-installable
+    // downgrade for anyone already on v0.5.2. The floor is raised to clear it.
+    it("clears the highest code shipped by earlier builds", () => {
       run("0.0.1");
-      expect(versionCode()).toBeGreaterThan(1000);
+      // v0.5.2 shipped 5002; anything at or below that cannot install over it.
+      expect(versionCode()).toBeGreaterThan(5002);
     });
 
-    it("uses 1000 + major*10000 + minor*100 + patch", () => {
+    it("keeps 0.5.3 installable over the 5002 that shipped as v0.5.2", () => {
+      run("0.5.3");
+      expect(versionCode()).toBeGreaterThan(5002);
+    });
+
+    it("uses 10000 + major*1000000 + minor*1000 + patch", () => {
       const cases: Array<[string, number]> = [
-        ["0.0.14", 1014],
-        ["0.0.15", 1015],
-        ["0.1.0", 1100],
-        ["0.4.8", 1408],
-        ["0.5.0", 1500],
-        ["1.0.0", 11000],
+        ["0.0.14", 10014],
+        ["0.0.15", 10015],
+        ["0.1.0", 11000],
+        ["0.4.8", 14008],
+        ["0.5.0", 15000],
+        ["0.5.3", 15003],
+        ["1.0.0", 1010000],
       ];
       for (const [version, code] of cases) {
         seed(tmp);
@@ -145,7 +157,7 @@ describe("set-version.sh", () => {
     // stripped before the arithmetic.
     it("derives the code from the numeric core of a prerelease", () => {
       run("0.6.0-rc1");
-      expect(versionCode()).toBe(1600);
+      expect(versionCode()).toBe(16000);
       expect(JSON.parse(read("package.json")).version).toBe("0.6.0-rc1");
     });
   });