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/routes/player/[id]/+page.svelte

@@ -451,7 +451,10 @@
     savedProgress = null;
     const id = itemId;
     if (id) {
-      loadAndPlay(id, 0);
+      // forceRestart bypasses the resume-progress check; without it, passing a
+      // start position of 0 is treated as "no position" (`!startPosition`), which
+      // re-runs the resume check and re-shows this very dialog in a loop.
+      loadAndPlay(id, 0, true);
     }
   }
 
@@ -537,7 +540,12 @@
     if (id) {
       reportPlaybackStopped(id, positionSeconds);
     }
-    html5Adapter.reportState("stopped", id ?? null);
+    // Intentionally do NOT emit a "stopped" player state here. This runs on both
+    // natural end-of-video (an autoplay handoff the backend's on_video_playback_ended
+    // owns) and on player close/unmount (where player_stop already drives the
+    // backend state). Emitting StateChanged{stopped} on natural end flips the
+    // player/mode to idle mid-handoff and suppresses next-episode auto-advance —
+    // the "sleep timer pauses at the end of an episode instead of continuing" bug.
   }
 
   async function handleVideoEnded() {