fix(player): advance background audio-only episodes in the backend (UR-040)

An episode played audio-only while the app was backgrounded stalled at the
episode boundary instead of advancing, and ExoPlayer parked in STATE_ENDED —
where any later play intent (lockscreen, headset, Bluetooth reconnect) replays
the ended item, surfacing as the episode randomly restarting.

End-of-playback is dispatched from two places and they disagreed. The Android
JNI callback carried the background-audio branch but can never reach it:
load_and_play sets EndReason::NewTrackLoaded at every load and nothing clears
it, so the first real end consumes it and the decision is always Stop. The call
that actually decides is the frontend's echo of the resulting PlaybackEnded into
player_on_playback_ended — and that path had no background-audio case at all, so
it started a countdown whose advance is a webview goto() that cannot start audio
while backgrounded.

Both dispatchers now share PlayerController::auto_advance_to_next_episode, so
they cannot drift apart again.

The handoff base offset moves from the BackgroundAudioOffset Tauri state onto
the controller, and the advance clears it: the next episode's stream is built
without StartTimeTicks, so its timeline is already absolute and a stale base
made player_exit_background_audio return old_base + position_in_new_episode.
Unreachable until the advance actually worked.

Tests (red before the fix):
- test_auto_advance_background_audio_episode_advances_in_backend
- test_auto_advance_foreground_video_episode_uses_countdown
- test_advance_to_next_episode_audio_only_clears_handoff_base

Bump to 0.2.9.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "jellytau",
-  "version": "0.2.8",
+  "version": "0.2.9",
   "description": "",
   "type": "module",
   "packageManager": "bun@1.3.5",