fix(autoplay): advance to the next episode in background audio mode

An episode handed off to the audio-only path for background playback is a
MediaType::Audio item, so autoplay's video-only checks stopped
recognising it as an episode: playback simply ended at the episode
boundary instead of continuing to the next one.

- Carry episode identity (item_type, series_id) through the
  background-audio handoff so the backend queue item still knows it's an
  episode; is_episode_item now trusts item_type over the media_type
  heuristic, and the sleep timer's episode counter follows.
- The frontend normally performs the advance by navigating to
  /player/<id>, which is unavailable while the WebView is suspended.
  advance_to_next_episode_audio_only drives it entirely in the backend:
  fetch the next episode, build its audio-only stream URL, and load it
  into the native audio player, preserving episode identity so the
  following boundary advances too.
- Android's autoplay dispatch routes background-audio episodes to that
  backend advance and keeps the countdown path for the foreground.
- get_audio_only_stream_url_for_video joins the MediaRepository trait
  (online delegates to the existing builder, offline errors) so the
  controller can reach it without a frontend round-trip.

TRACES: UR-040, UR-023 | DR-052 | JA-032

src-tauri/src/repository/offline.rs

@@ -1518,6 +1518,17 @@ impl MediaRepository for OfflineRepository {
         Err(RepoError::Offline)
     }
 
+    async fn get_audio_only_stream_url_for_video(
+        &self,
+        _item_id: &str,
+        _media_source_id: Option<&str>,
+        _start_time_seconds: Option<f64>,
+        _audio_stream_index: Option<i32>,
+    ) -> Result<String, RepoError> {
+        // Audio-only transcode requires the server; offline downloads play locally.
+        Err(RepoError::Offline)
+    }
+
     async fn get_live_tv_channels(&self) -> Result<Vec<MediaItem>, RepoError> {
         // Live TV is inherently online-only.
         Err(RepoError::Offline)