Phase 4: Media3 playback engine

Builds the single MediaSession-backed player that in-app playback,
offline downloads (Phase 5), and Android Auto (Phase 6) will all
share, per the Phase 2+ roadmap's Phase 4.

- DeepwavePlaybackService: ExoPlayer + MediaLibrarySession, built as
  a MediaLibraryService from day one (not the plainer
  MediaSessionService) so Android Auto only has to add browse-tree
  content later, never rebuild the service. Browse tree is a stub
  for now (MediaLibrarySession.Callback's defaults deny browsing).
- LocalOrRemoteDataSource: the entire "prefer a downloaded file over
  streaming" mechanism as a ResolvingDataSource.Resolver, backed by
  an in-memory LocalTrackFiles registry that stays empty until
  Phase 5 populates it from Room - reused unmodified by Android Auto
  later.
- ExoPlayer's OkHttpDataSource shares the same signed OkHttpClient as
  Retrofit/Coil, so SubsonicRequestInterceptor signs stream requests
  identically to every other Subsonic call - streamUrl() builds the
  request URL the same way coverArtUrl() does for Coil.
- PlaybackController: app-facing facade over a MediaController,
  exposing Flow<PlaybackUiState> - same facade convention as
  ServerRepository. NowPlayingScreen (new, full-screen: artwork,
  seek bar, play/pause/skip) and MiniPlayerBar (now real, replacing
  Phase 2/3's static placeholder) both consume it.
- Track clicks in AlbumDetailScreen (queues the whole album from the
  clicked index) and SearchScreen (single-song queue) now actually
  play, via PlaybackController injected into their ViewModels.
- Scrobbling: submission=false when a track starts, submission=true
  for the outgoing track on each transition - a simplified heuristic
  rather than a played-percentage threshold, feeding Navidrome's
  play-count data that Phase 3's getAlbumList2(frequent/recent) rows
  read from.
- Runtime POST_NOTIFICATIONS request added to MainActivity for API
  33+ (declared in the manifest but easy to forget the runtime half
  of - without it the service still plays, but its notification
  never shows).

Split MainScreen into a thin PlayerViewModel-resolving wrapper plus a
previewable MainContent, rather than having MainScreen call
hiltViewModel() directly - preserves the interactive Android Studio
preview from last session, which a direct Hilt dependency would have
broken.

media3 pinned to 1.11.1 (built against Kotlin 2.2.0, matching this
project's 2.2.10 - learned from Phase 3's Coil version conflict to
check this before picking a version this time).

Verified extensively on-device against the real Navidrome server:
real tracks play with correct metadata/artwork, pause/resume and
queue auto-advance work, the real Android MediaSession exposes
correct state (checked independently via `dumpsys media_session`,
not just the app's own UI), background playback survives
foregrounding/backgrounding, and no crashes across the whole session.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/src/main/res/values/strings.xml

@@ -20,6 +20,7 @@
 
     <string name="player_nothing_playing">Nothing playing</string>
     <string name="player_play">Play</string>
+    <string name="player_pause">Pause</string>
 
     <string name="home_row_recently_added">Recently Added</string>
     <string name="home_row_recently_played">Recently Played</string>