Introduce PlayerAdapter contract; decision logic shared in Rust backend

Establish a decoupled player boundary so UI and backend interact with video
through one contract, with the HTML5 (Linux/interim-Android) and native
(ExoPlayer) providers as interchangeable primitive-executor adapters.

- PlayerAdapter interface + AdapterHost callback bag (adapters/types.ts): the
  adapter owns only decision-free element PRIMITIVES (seekElement, reloadSource,
  play/pause, setVolume, selectSubtitle); it never branches on strategy.
- Seek/audio-track DECISIONS stay in Rust (player_seek_video / _switch_audio_track
  return a strategy); the facade dispatches the chosen primitive to the active
  adapter. Both providers share the one decision path — logic lives once, in Rust.
- Facade holds the active adapter; a new ControlCommand PlayerStatusEvent lets
  backend control (lockscreen/remote/sleep) drive the webview <video> element.
- Html5PlayerAdapter resolves the LIVE element via the bridge (fixes play/pause
  silently no-opping when the element was re-bound).
- Do not emit a "stopped" player state on natural end-of-video: it flipped the
  player/mode to idle mid-handoff and suppressed next-episode auto-advance under
  a sleep timer. Jellyfin progress reporting is preserved; the backend's
  on_video_playback_ended owns the transition.
- VideoPlayer net -300 lines (strategy/HLS-reload logic relocated to the adapter).
- Adds 20 adapter unit tests; existing suites stay green (vitest 457, cargo 416).

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

src-tauri/src/player/events.rs

@@ -130,6 +130,17 @@ pub enum PlayerStatusEvent {
     /// frontend owns the two-step remote->local transfer (it must reload the
     /// media item locally), so the native side only signals intent here.
     RemoteDisconnectRequested,
+    /// Backend-originated control command targeting the active frontend player
+    /// adapter (the HTML5 <video> that lives in the webview, which Rust cannot
+    /// drive directly). Emitted by control paths like the sleep timer, lockscreen,
+    /// or remote so they can pause/play/seek/stop the webview element.
+    /// `playerEvents.ts` routes this to the active PlayerAdapter via the facade.
+    ControlCommand {
+        /// One of: "play", "pause", "stop", "seek".
+        action: String,
+        /// Target position in seconds (only meaningful for "seek").
+        position: Option<f64>,
+    },
 }
 
 /// Trait for emitting player events to the frontend.