fix(android): keep lockscreen/media controls in sync with playback

The lockscreen controls drifted out of sync, especially while casting, and
couldn't control remote playback. Two media sessions were competing (a Media3
MediaSession driving transport vs a MediaSessionCompat driving the notification),
position was only pushed on play/pause so the scrubber froze mid-track, and
remote mode showed stale local metadata with dead buttons.

- Make MediaSessionCompat the single source of truth; route all transport
  commands (both the Compat callback and the Media3 wrappedPlayer) through Rust
  via nativeOnMediaCommand instead of touching ExoPlayer directly.
- Push position on every 250ms tick via a lightweight updatePlaybackPosition,
  and report 0.0 playback speed when paused so Android stops extrapolating.
- Mirror the remote session's now-playing onto the lockscreen from the native
  session poller (works while the screen is locked, unlike WebView timers) via
  a new player::update_lockscreen_metadata JNI bridge.
- Make MediaSessionHandler mode-aware: in remote mode forward play/pause/next/
  prev/seek to the remote Jellyfin session; Stop while casting emits
  RemoteDisconnectRequested, which the frontend handles by transferring to local.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src-tauri/src/player/events.rs

@@ -119,6 +119,12 @@ pub enum PlayerStatusEvent {
         /// All active controllable sessions from Jellyfin
         sessions: Vec<crate::jellyfin::client::SessionInfo>,
     },
+    /// The user asked to disconnect from the remote session and resume locally.
+    ///
+    /// Emitted when the lockscreen Stop button is pressed while casting. The
+    /// frontend owns the two-step remote->local transfer (it must reload the
+    /// media item locally), so the native side only signals intent here.
+    RemoteDisconnectRequested,
 }
 
 /// Trait for emitting player events to the frontend.