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/services/playerEvents.ts

@@ -17,6 +17,7 @@ import { sleepTimer, sleepTimerExpiredSignal } from "$lib/stores/sleepTimer";
 import { nextEpisode, nextEpisodeItem as nextEpisodeItemStore } from "$lib/stores/nextEpisode";
 import { autoPlayNext } from "$lib/services/nextEpisodeService";
 import { preloadUpcomingTracks } from "$lib/services/preload";
+import { playerController } from "$lib/player";
 import type { MediaItem } from "$lib/api/types";
 import { get } from "svelte/store";
 
@@ -116,9 +117,20 @@ function handlePlayerEvent(event: PlayerStatusEvent): void {
     case "sleep_timer_expired":
       // Backend stops its own playback; this signal lets HTML5 video (which
       // plays outside the backend on Linux) pause itself too.
+      // Preferred path: drive the active video adapter directly so the backend
+      // has real control authority over the webview element. The legacy
+      // sleepTimerExpiredSignal is kept for any remaining subscribers.
+      playerController.getActiveAdapter()?.pause();
       sleepTimerExpiredSignal.update((n) => n + 1);
       break;
 
+    case "control_command":
+      // Backend-originated control targeting the active frontend player adapter
+      // (lockscreen/remote/sleep). Route it to the adapter so a backend intent
+      // reaches the webview <video> element.
+      handleControlCommand(event.action, event.position);
+      break;
+
     case "show_next_episode_popup":
       handleShowNextEpisodePopup(
         event.current_episode,
@@ -292,6 +304,36 @@ function handleSleepTimerChanged(mode: SleepTimerMode, remainingSeconds: number)
   sleepTimer.set({ mode, remainingSeconds });
 }
 
+/**
+ * Route a backend-originated control command to the active player adapter, so a
+ * backend intent (lockscreen/remote/sleep) can drive the webview <video> element
+ * that Rust cannot reach directly. No-op when no video adapter is active (audio
+ * playback is already fully backend-driven).
+ */
+function handleControlCommand(action: string, position: number | null): void {
+  const adapter = playerController.getActiveAdapter();
+  if (!adapter) return;
+  switch (action) {
+    case "play":
+      void adapter.play();
+      break;
+    case "pause":
+      void adapter.pause();
+      break;
+    case "seek":
+      // Backend-driven in-place seek (e.g. lockscreen scrub). The backend has
+      // already decided this is a simple position change, so use the element
+      // seek primitive with no transcode offset.
+      if (position != null) void adapter.seekElement(position, 0);
+      break;
+    case "stop":
+      void adapter.pause();
+      break;
+    default:
+      console.warn("[playerEvents] Unknown control command:", action);
+  }
+}
+
 /**
  * Handle show next episode popup event.
  *