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>

android/app/src/main/AndroidManifest.xml

@@ -5,7 +5,7 @@
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE"/>
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE_MEDIA_PLAYBACK"/>
-    <uses-permission android:name="android.permission.POST_NOTIFICATIONS"/>
+<uses-permission android:name="android.permission.POST_NOTIFICATIONS"/>
     <uses-permission android:name="android.permission.WAKE_LOCK"/>
     <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED"/>