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.sh

@@ -81,8 +81,16 @@ fi
 # builds shipped versionCode 1000 (from a 0.1.0 config), so a plain 15 is a
 # *downgrade* and Android refuses the update.
 #
-#   code = 1000 + major*10000 + minor*100 + patch
-# e.g. 0.0.14 -> 1014, 0.0.15 -> 1015, 0.1.0 -> 1100, 1.0.0 -> 11000.
+# The floor has to clear 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** under an earlier `minor*1000` scheme; the `minor*100` formula that
+# replaced it yields only 1502 for that same 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, which is exactly the failure this block exists to prevent.
+# The multipliers are widened and the floor raised past 5002 accordingly.
+#
+#   code = 10000 + major*1000000 + minor*1000 + patch
+# e.g. 0.0.14 -> 10014, 0.1.0 -> 11000, 0.5.3 -> 15003, 1.0.0 -> 1010000.
 PROPS="src-tauri/gen/android/app/tauri.properties"
 if [ -f "$PROPS" ]; then
   # Strip any -rc1/+build suffix first: it is not numeric, and feeding it to
@@ -93,7 +101,7 @@ if [ -f "$PROPS" ]; then
   MIN=$(echo "$CORE" | cut -d. -f2)
   PAT=$(echo "$CORE" | cut -d. -f3)
   : "${MAJ:=0}" "${MIN:=0}" "${PAT:=0}"
-  CODE=$(( 1000 + MAJ*10000 + MIN*100 + PAT ))
+  CODE=$(( 10000 + MAJ*1000000 + MIN*1000 + PAT ))
   echo "   versionCode=$CODE (from $CORE)"
   if grep -q '^tauri.android.versionCode=' "$PROPS"; then
     sed -i "s/^tauri.android.versionCode=.*/tauri.android.versionCode=$CODE/" "$PROPS"