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/lib/api/bindings.ts

@@ -1996,7 +1996,15 @@ export type PlayerStatusEvent =
  * frontend owns the two-step remote->local transfer (it must reload the
  * media item locally), so the native side only signals intent here.
  */
-{ type: "remote_disconnect_requested" }
+{ type: "remote_disconnect_requested" } | 
+/**
+ * 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.
+ */
+{ type: "control_command"; action: string; position: number | null }
 /**
  * Result of creating a playlist
  *