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/build.gradle.kts

@@ -87,6 +87,9 @@ dependencies {
     implementation(libs.room.ktx)
     ksp(libs.room.compiler)
 
+    // WorkManager
+    implementation(libs.work.runtime.ktx)
+
     // DataStore
     implementation(libs.datastore.preferences)