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>

README.md

@@ -7,70 +7,17 @@ Built with Vite + vanilla ES modules (web) and Jetpack Compose + Media3/ExoPlaye
 
 ## Features
 
-### Music Library
-- Stream music from your Navidrome / Subsonic library
-- Browse artists, albums, playlists, starred tracks, genres, and random picks
-- Search across songs, albums, and artists
-- Star and context menu on every song row
-- Play count display (optional)
+- **Library** — Stream from Navidrome / Subsonic; browse artists, albums, playlists, genres, starred, and random; search; star + context menu on every row.
+- **Playback** — Queue with shuffle, repeat, and radio mode (endless similar songs); gapless-style preload; lock-screen / notification controls; Navidrome scrobbling.
+- **Lyrics** — Synced and plain lyrics via lrclib.net, auto-fetched and cached per session.
+- **Artist pages** — Hero card, Last.fm bio / similar / top songs, Ticketmaster events, and iTunes cross-reference showing owned vs. missing tracks.
+- **Playlists** — Create, rename, delete, add/remove tracks, and upload a cover (Web + Android).
+- **Importer & downloads** — Paste a Spotify / YouTube / YouTube Music / SoundCloud link to download via yt-dlp, with iTunes metadata enrichment and duplicate detection.
+- **Metadata editor** (web) — Edit tags + cover art; files are renamed/moved and Navidrome is rescanned on save.
+- **Offline** (Android) — Download albums/playlists to private storage; an Offline tab to play and manage them; downloaded tracks play from disk on every screen.
+- **Audiobookshelf** — Browse and stream audiobooks with chapter navigation, speed control, sleep timer, and progress sync.
 
-### Lyrics
-- Synced and plain lyrics via lrclib.net, auto-fetched per track
-- Fallback chain: exact match → no-duration match → fuzzy artist+title search → title-only search
-- In-memory cache per session
-
-### Artist Pages
-- Hero card with artist photo and blurred background
-- Bio, similar artists, top songs from Last.fm
-- Concert events via Ticketmaster
-- Albums cross-referenced against iTunes catalog — shows downloaded vs. missing tracks
-- Artist cards with images in search results
-
-### Playback
-- Queue with shuffle, repeat (all / one), and radio mode (endless similar songs)
-- Now Playing overlay with synced lyrics, queue view, star, and multi-artist links
-- Gapless-style playback — next track pre-buffered before current track ends
-- Lock screen / notification media controls
-- Scrobbling to Navidrome (configurable delay)
-
-### Discovery
-- iTunes / Last.fm — charts, global search, 30-second previews, similar artists
-- Song Importer — paste a Spotify, YouTube, YouTube Music, or SoundCloud link to download
-
-### Downloads
-- Download tracks, albums, or entire artists from YouTube Music / SoundCloud / Spotify via yt-dlp
-- iTunes metadata enrichment — album art, year, track number, genre
-- Duplicate detection, standard folder/filename structure
-
-### Metadata Editor
-- Edit tags (title, artist, album, genre, year, track, disc, cover art)
-- File renamed and moved automatically; Navidrome rescan triggered on save
-
-### Playlists
-- Create, rename, and delete playlists
-- Add tracks via "Add to Playlist" context menu
-- Remove tracks from a playlist (edit mode on Android; inline on Web)
-- Cover image upload (Web + Android)
-
-### Audiobookshelf Integration
-- Browse libraries and audiobooks on both Web and Android
-- Stream audiobooks with chapter-by-chapter navigation
-- Playback speed control (0.5× – 2.0×)
-- Sleep timer
-- Progress sync — current position saved back to ABS server every 30 s
-- Chapter track mode — progress bar shows current chapter length instead of full book (toggle in Settings)
-- Tab/section hidden until ABS is configured in Settings
-
-### App — Web PWA
-- Installable on Android / iOS / desktop
-- Multiple color themes, German UI
-
-### App — Android (native Jetpack Compose)
-- Media3 / ExoPlayer backend
-- OS notification with playback controls
-- ABS audiobook notification: chapter name shown as title, book title as artist; extra buttons — Kapitel zurück | 30s zurück | Play/Pause | 30s vor | Kapitel vor (compact + expanded)
-- Last played song shown in mini player bar on restart; tap to resume
-- Full playlist management: create, rename (with description), delete, remove tracks, cover upload
+Web is an installable PWA (themes, German UI); Android is native Jetpack Compose + Media3/ExoPlayer with a mini player, media/ABS notifications, and Android Auto. See [CHANGELOG.md](CHANGELOG.md) for the full web/Android parity matrix.
 
 ---
 
@@ -119,6 +66,7 @@ NovaSonic/
 │   └── app/src/main/kotlin/com/novasonic/app/
 │       ├── api/               # SubsonicClient, AbsClient, LrcLibClient, ItunesClient
 │       ├── data/              # PrefsStore (DataStore), MusicRepository
+│       ├── offline/           # Room DB, DAO, OfflineRepository, DownloadWorker (offline downloads)
 │       ├── player/            # PlayerController, MusicService (Media3)
 │       ├── ui/
 │       │   ├── screen/        # HomeScreen, NowPlayingScreen, AbsScreen, SettingsScreen, …