feat(library): focused music/TV/movie landing screens + self-draining download queue

Library screens:
- Add dedicated music, TV, and movie landing pages (hero banner +
  horizontal carousels) backed by new music/tv/movies stores.
- Route tvshows libraries to /library/tv; surface rediscover ("haven't
  listened to in a while") albums via a new repository method across
  online/offline/hybrid repos plus the repository_get_rediscover_albums
  command.
- Add an A-Z jump bar for long alphabetically-sorted lists, with grid
  index anchors in LibraryGrid/LibraryListView/TrackList.
- Filter the "Podcasts" folder out of music library queries.

Downloads:
- Add a backend queue pump: enqueue_download / enqueue_video_downloads
  persist the resolved stream URL + target dir on each row (migration
  017), and the pump starts up to max_concurrent and drains the rest
  automatically as slots free, instead of the frontend silently dropping
  items past the concurrency limit. Album/series/season buttons now
  enqueue rather than calling start_download directly.

Other fixes:
- Hybrid search now returns instant cache results and pushes the merged
  cache+server union via a request-id-tagged search-event, so superseded
  queries can't clobber fresher results.
- URL-encode SearchTerm / genres / item types in online repo requests.
- Android: pause on audio-becoming-noisy (headphone/BT disconnect).

src-tauri/src/repository/offline.rs

@@ -778,6 +778,17 @@ impl MediaRepository for OfflineRepository {
         Ok(items)
     }
 
+    async fn get_rediscover_albums(
+        &self,
+        _parent_id: Option<&str>,
+        _limit: Option<usize>,
+    ) -> Result<Vec<MediaItem>, RepoError> {
+        // "Rediscover" is a discovery feature over the full server library.
+        // Offline only holds downloaded items, so there is nothing meaningful
+        // to surface here; the hybrid repo serves this from the server instead.
+        Ok(Vec::new())
+    }
+
     async fn get_resume_movies(&self, limit: Option<usize>) -> Result<Vec<MediaItem>, RepoError> {
         let limit_val = limit.unwrap_or(12);