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>

CHANGELOG.md

@@ -45,11 +45,33 @@ Versioning: two-part `MAJOR.MINOR` — each release bumps the minor (features an
 | Media notification               | ✅        | ✅      |
 | ABS notification (modern style)  | ❌        | ✅      |
 | Android Auto                     | —         | ✅      |
+| Offline download (albums/playlists)  | ❌        | ✅      |
+| Offline playback                     | ❌        | ✅      |
+| Offline detail / manage downloads    | ❌        | ✅      |
 | Romaji / Japanese sort           | ✅        | ❌      |
 | Settings (Navidrome, ABS, etc.)  | ✅        | ✅      |
 
 ---
 
+## [0.10] — 2026-06-25
+
+### Added
+- **Android** — Offline downloads: download albums and playlists to device storage with a single tap; album download button is in the album hero row (▶ / shuffle / ↓), playlist button next to the edit toggle. Tapping again resumes / re-downloads any missing songs (already-downloaded tracks are skipped)
+- **Android** — Offline tab in Library (position 2, next to Playlists): lists downloaded groups with cover art, track count, and an in-progress bar; shows total MB used
+- **Android** — Offline detail screen: tap a group to view its songs (with cover art, like the online playlist), play / shuffle, and remove individual songs from the device via the edit toggle. Works fully offline (reads from Room, no network)
+- **Android** — Offline playback: tap ▶ on a fully-downloaded group to play locally; downloaded songs are also served from disk automatically when opened from any album or playlist view
+- **Android** — Downloaded badge: albums and playlists in the Library tabs show a ↓-in-circle badge on the cover when fully downloaded
+- **Android** — Duplicate-safe storage: a song shared by an album and a playlist is stored once (keyed by song ID) and reference-counted on delete — removing one group keeps the file if another still needs it
+
+### Fixed
+- **Android** — Download crash on Android 15 (targetSdk 35): `FOREGROUND_SERVICE_TYPE_DATA_SYNC` is banned for apps targeting API 35; WorkManager's expedited path used it and the resulting exception killed the app ~1.5 s after tapping download. Fixed by dropping `setExpedited()` and all foreground-service use from `DownloadWorker` — downloads run as silent background work with progress shown in the Offline tab
+- **Android** — Offline detail screen flashed open then immediately returned: its initial empty state (before Room emitted) was mistaken for "deleted". Now shows a brief spinner until loaded, and only leaves when the last song is actually removed
+
+### Internal
+- **Android** — Offline system: `Room` DB (`novasonic_offline.db`) with `downloaded_tracks`, `download_groups`, `group_track_cross_refs`; `OfflineRepository` holds hot `StateFlow` maps for O(1) local-path lookups; `DownloadWorker` streams audio + cover art via `HttpURLConnection` to `context.filesDir/offline/{audio|covers}/`. `MusicRepository.streamUrl()` checks local files first, so playback uses the `file://` URI on every entry point
+
+---
+
 ## [0.9] — 2026-06-22
 
 ### Added