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>

src/backend/qobuzbackend.cpp

@@ -10,7 +10,7 @@ QobuzBackend::QobuzBackend(QObject *parent)
     m_backend = qobuz_backend_new(&QobuzBackend::eventTrampoline, this);
 
     m_positionTimer = new QTimer(this);
-    m_positionTimer->setInterval(500);
+    m_positionTimer->setInterval(50);
     connect(m_positionTimer, &QTimer::timeout, this, &QobuzBackend::onPositionTick);
     m_positionTimer->start();
 }
@@ -79,6 +79,40 @@ void QobuzBackend::getUserPlaylists(quint32 offset, quint32 limit)
     qobuz_backend_get_user_playlists(m_backend, offset, limit);
 }
 
+// ---- playback options ----
+
+void QobuzBackend::setReplayGain(bool enabled)
+{
+    qobuz_backend_set_replaygain(m_backend, enabled);
+}
+
+void QobuzBackend::prefetchTrack(qint64 trackId, int formatId)
+{
+    qobuz_backend_prefetch_track(m_backend, trackId, formatId);
+}
+
+// ---- playlist management ----
+
+void QobuzBackend::createPlaylist(const QString &name)
+{
+    qobuz_backend_create_playlist(m_backend, name.toUtf8().constData());
+}
+
+void QobuzBackend::deletePlaylist(qint64 playlistId)
+{
+    qobuz_backend_delete_playlist(m_backend, playlistId);
+}
+
+void QobuzBackend::addTrackToPlaylist(qint64 playlistId, qint64 trackId)
+{
+    qobuz_backend_add_track_to_playlist(m_backend, playlistId, trackId);
+}
+
+void QobuzBackend::deleteTrackFromPlaylist(qint64 playlistId, qint64 playlistTrackId)
+{
+    qobuz_backend_delete_track_from_playlist(m_backend, playlistId, playlistTrackId);
+}
+
 // ---- fav modification ----
 
 void QobuzBackend::addFavTrack(qint64 trackId)
@@ -201,6 +235,12 @@ void QobuzBackend::onEvent(int eventType, const QString &json)
     case EV_STATE_CHANGED:
         emit stateChanged(obj["state"].toString());
         break;
+    case 20: // EV_PLAYLIST_CREATED
+        emit playlistCreated(obj);
+        break;
+    case 21: // EV_PLAYLIST_DELETED
+        emit playlistDeleted(obj);
+        break;
     case EV_GENERIC_ERR:
     case EV_TRACK_URL_ERR:
         emit error(obj["error"].toString());