feat: playlist management, gapless playback, ReplayGain, Qobuz theme

Playlist management:
- Add/remove tracks from playlists via right-click context menu
- Create new playlists (right-click Playlists sidebar header)
- Delete playlists with confirmation dialog (right-click playlist item)
- Playlist view removes track immediately on delete (optimistic)
- Deleting currently-open playlist clears the track view

Gapless playback:
- Single long-running audio thread owns AudioOutput; CPAL stream stays
  open between tracks eliminating device teardown/startup gap
- Decode runs inline on the audio thread; command channel polled via
  try_recv() so Pause/Resume/Seek/Stop/Play all work without spawning
- New Play command arriving mid-decode is handled immediately,
  reusing the same audio output for zero-gap transition
- Position timer reduced from 500 ms to 50 ms for faster track-end detection
- URL/metadata prefetch: when gapless is enabled Qt pre-fetches the next
  track while the current one is still playing

ReplayGain:
- Toggled in Settings → Playback
- replaygain_track_gain (dB) from track audio_info converted to linear
  gain factor and applied per-sample alongside volume

Qobuz dark theme:
- Background #191919, base #141414, accent #FFB232 (yellow-orange)
- Selection highlight, slider fill, scrollbar hover all use #FFB232
- Links use Qobuz blue #46B3EE
- Hi-res H badges updated to #FFB232 (from #FFD700)
- Now-playing row uses #FFB232 (was Spotify green)
- QSS stylesheet for scrollbars, menus, inputs, buttons, groups

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

rust/include/qobuz_backend.h

@@ -30,7 +30,9 @@ enum QobuzEvent {
     EV_POSITION       = 16,
     EV_TRACK_URL_OK   = 17,
     EV_TRACK_URL_ERR  = 18,
-    EV_GENERIC_ERR    = 19,
+    EV_GENERIC_ERR      = 19,
+    EV_PLAYLIST_CREATED = 20,
+    EV_PLAYLIST_DELETED = 21,
 };
 
 // Callback signature
@@ -69,6 +71,16 @@ uint8_t  qobuz_backend_get_volume(const QobuzBackendOpaque *backend);
 int      qobuz_backend_get_state(const QobuzBackendOpaque *backend);
 int      qobuz_backend_take_track_finished(QobuzBackendOpaque *backend);
 
+// ReplayGain / Gapless
+void qobuz_backend_set_replaygain(QobuzBackendOpaque *backend, bool enabled);
+void qobuz_backend_prefetch_track(QobuzBackendOpaque *backend, int64_t track_id, int32_t format_id);
+
+// Playlist management
+void qobuz_backend_create_playlist(QobuzBackendOpaque *backend, const char *name);
+void qobuz_backend_delete_playlist(QobuzBackendOpaque *backend, int64_t playlist_id);
+void qobuz_backend_add_track_to_playlist(QobuzBackendOpaque *backend, int64_t playlist_id, int64_t track_id);
+void qobuz_backend_delete_track_from_playlist(QobuzBackendOpaque *backend, int64_t playlist_id, int64_t playlist_track_id);
+
 // Favorites modification
 void qobuz_backend_add_fav_track(QobuzBackendOpaque *backend, int64_t track_id);
 void qobuz_backend_remove_fav_track(QobuzBackendOpaque *backend, int64_t track_id);