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-tauri/src/lib.rs

@@ -265,6 +265,7 @@ use commands::{
     storage_save_user,
     storage_search_items,
     storage_set_active_user,
+    storage_set_watched,
     storage_toggle_favorite,
     storage_update_playback_context,
     storage_update_playback_progress,
@@ -424,6 +425,28 @@ impl MediaSessionHandler {
 
     /// Drive the local player for a transport command.
     fn handle_local_command(&self, command: &str) {
+        // A lockscreen scrub is an ABSOLUTE position — the scrubber shows the
+        // whole episode — and resolving it during a background-audio handoff means
+        // re-opening the stream, which is async. So it runs on the runtime and,
+        // critically, is handled *before* the blocking lock below: taking that
+        // guard and then spawning a task that waits for the same mutex would
+        // deadlock the media session. (DR-159)
+        if let Some(raw) = command.strip_prefix("seek:") {
+            match raw.parse::<f64>() {
+                Ok(position) => {
+                    let player = self.player.clone();
+                    tokio::spawn(async move {
+                        let controller = player.lock().await;
+                        if let Err(e) = controller.seek_absolute(position).await {
+                            error!("[MediaSession] Seek to {:.1}s failed: {}", position, e);
+                        }
+                    });
+                }
+                Err(_) => warn!("[MediaSession] Bad seek command: {}", command),
+            }
+            return;
+        }
+
         // Use blocking_lock since this is called from a non-async JNI callback
         let controller = self.player.blocking_lock();
 
@@ -433,13 +456,6 @@ impl MediaSessionHandler {
             "next" => controller.next(),
             "previous" => controller.previous(),
             "stop" => controller.stop(),
-            cmd if cmd.starts_with("seek:") => match cmd[5..].parse::<f64>() {
-                Ok(pos) => controller.seek(pos),
-                Err(_) => {
-                    warn!("[MediaSession] Bad seek command: {}", command);
-                    Ok(())
-                }
-            },
             _ => {
                 warn!("[MediaSession] Unknown command: {}", command);
                 Ok(())
@@ -789,6 +805,7 @@ fn specta_builder() -> Builder<tauri::Wry> {
             storage_update_playback_progress,
             storage_update_playback_context,
             storage_mark_played,
+            storage_set_watched,
             storage_get_playback_progress,
             storage_mark_synced,
             storage_toggle_favorite,