feat(android): offline downloads — albums, playlists, detail screen & playback

Download albums/playlists to private device storage (Room + WorkManager,
HttpURLConnection streaming). New Offline tab lists groups with progress;
tapping a group opens a detail screen (cover-art rows, play/shuffle, per-song
delete from device). Downloaded tracks play from disk on every screen.
Album/playlist cards show a downloaded badge; download button resumes missing
songs. Song storage is dedup'd by ID and reference-counted on delete.

Fixes: Android 15 download crash (dropped banned DATA_SYNC foreground service);
offline detail flash-back (initial empty state no longer mistaken for deleted).

Docs: CHANGELOG 0.10 + parity matrix; condensed README Features.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -70,6 +70,15 @@ Backend runs in Docker (see `docker-compose.yml`); on the deploy host it lives a
   share the same `mainArtist` value (= iTunes `collectionArtistName` when available, else
   `albumArtist` param, else track artist). Never use `rawArtist` for `album_artist`.
 
+## Android offline system
+
+- **Storage:** `context.filesDir/offline/audio/{songId}.{ext}` and `.../covers/{coverArtId}.jpg` — private internal storage, not accessible via file manager.
+- **Database:** Room DB `novasonic_offline.db` (version 1) with three tables: `downloaded_tracks` (track metadata + local paths + state), `download_groups` (album/playlist/song group + progress), `group_track_cross_refs` (many-to-many). See [OfflineEntities.kt](android/app/src/main/kotlin/com/novasonic/app/offline/OfflineEntities.kt) and [OfflineDao.kt](android/app/src/main/kotlin/com/novasonic/app/offline/OfflineDao.kt).
+- **OfflineRepository** ([OfflineRepository.kt](android/app/src/main/kotlin/com/novasonic/app/offline/OfflineRepository.kt)): holds hot `StateFlow` maps (`_audioMap`, `_coverMap`) keyed by songId/coverArtId for O(1) lookups in `MusicRepository.streamUrl()`. Call `enqueueGroup()` to schedule a download.
+- **DownloadWorker** ([DownloadWorker.kt](android/app/src/main/kotlin/com/novasonic/app/offline/DownloadWorker.kt)): `CoroutineWorker` using plain `HttpURLConnection` streaming to `.tmp` → rename. **No `setExpedited()` and no `setForeground()`** — `FOREGROUND_SERVICE_TYPE_DATA_SYNC` is banned on Android 15 (targetSdk=35) and crashes the app; downloads are silent background work.
+- **Playback:** `MusicRepository.streamUrl()` checks `offlineRepo.localAudioPath(id)` first; if the file exists it returns a `file://` URI. This applies to every play path — album, playlist, Offline tab — no special offline mode needed.
+- **Offline tab:** Library → tab 2 (OFFLINE). Driven by `OfflineViewModel`. Play button on a completed group calls `offlineVm.songsForGroup()` (JOIN query → `DownloadedTrackEntity.toSong()`) then `player.playSongs()`.
+
 ## UI / references
 
 When referencing files in chat, use clickable markdown links, not backticks: