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.

src-tauri/src/commands/player/mod.rs

@@ -57,18 +57,6 @@ pub struct MediaSessionManagerWrapper(pub Mutex<MediaSessionManager>);
 /// @req: DR-048 - Video settings (auto-play toggle, countdown duration)
 pub struct VideoSettingsWrapper(pub Mutex<VideoSettings>);
 
-/// Base offset (seconds) for the active background-audio handoff.
-///
-/// The audio-only stream is requested with `StartTimeTicks` = the handoff
-/// position, so the server makes that point the stream's zero. ExoPlayer then
-/// reports position RELATIVE to that zero. To convert back to an absolute
-/// position on exit (so the video resumes where the audio actually reached), we
-/// add this stored base to the native player's reported position.
-///
-/// TRACES: UR-040 | DR-052
-#[derive(Default)]
-pub struct BackgroundAudioOffset(pub Mutex<f64>);
-
 /// Response for player state queries
 #[derive(specta::Type, Debug, Serialize)]
 #[serde(rename_all = "camelCase")]
@@ -586,7 +574,6 @@ pub async fn player_play_item(
 pub async fn player_enter_background_audio(
     player: State<'_, PlayerStateWrapper>,
     session: State<'_, MediaSessionManagerWrapper>,
-    bg_offset: State<'_, BackgroundAudioOffset>,
     item: PlayItemRequest,
     position_seconds: f64,
 ) -> Result<PlayerStatus, String> {
@@ -636,17 +623,18 @@ pub async fn player_enter_background_audio(
         session_mgr.start_audio_session(media_item.clone());
     }
 
-    // Remember where the video was: the audio stream's zero == this position
-    // (the URL was built with StartTimeTicks=position_seconds), so on exit we add
-    // this base to the native player's relative position to get the absolute one.
-    *bg_offset.0.lock().map_err(|e| e.to_string())? = position_seconds.max(0.0);
-
     // Same base offset drives the lockscreen scrubber: ExoPlayer reports position
     // relative to the stream's StartTimeTicks zero, but the metadata duration is
     // absolute, so shift the reported position back to absolute for the scrubber.
     let _ = crate::player::set_lockscreen_position_offset(position_seconds.max(0.0));
 
     let controller = player.0.lock().await;
+    // Remember where the video was: the audio stream's zero == this position
+    // (the URL was built with StartTimeTicks=position_seconds), so on exit we add
+    // this base to the native player's relative position to get the absolute one.
+    // The controller owns it so a backend-driven advance to the next episode
+    // clears it along with the stream it described.
+    controller.set_background_audio_base(position_seconds);
     controller
         .play_item(media_item)
         .map_err(|e| e.to_string())?;
@@ -677,21 +665,15 @@ pub async fn player_enter_background_audio(
 #[specta::specta]
 pub async fn player_exit_background_audio(
     player: State<'_, PlayerStateWrapper>,
-    bg_offset: State<'_, BackgroundAudioOffset>,
 ) -> Result<f64, String> {
-    // The base offset (handoff position) + native player's relative position =
-    // the absolute position to resume the video at. Read/reset the base first.
-    let base = {
-        let mut off = bg_offset.0.lock().map_err(|e| e.to_string())?;
-        let b = *off;
-        *off = 0.0;
-        b
-    };
-
     // Back to foreground playback: the lockscreen scrubber is absolute again.
     let _ = crate::player::set_lockscreen_position_offset(0.0);
 
     let controller = player.0.lock().await;
+    // The base offset (handoff position) + native player's relative position =
+    // the absolute position to resume the video at. Zero after a backend-driven
+    // episode advance, whose stream already starts at its own zero.
+    let base = controller.take_background_audio_base();
     // Capture position into a `let` BEFORE stop() — never hold work across a lock
     // re-entrant call (deadlock discipline, CLAUDE.md).
     let relative = controller.position();