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/list/tracks.hpp

@@ -7,6 +7,9 @@
 #include <QTreeView>
 #include <QJsonArray>
 #include <QJsonObject>
+#include <QVector>
+#include <QPair>
+#include <QString>
 
 namespace List
 {
@@ -25,13 +28,22 @@ namespace List
         /// Called when the backend fires EV_TRACK_CHANGED so the playing row is highlighted.
         void setPlayingTrackId(qint64 id);
 
+        /// Set which playlist is currently displayed (0 = none).
+        void setPlaylistContext(qint64 playlistId);
+        /// Provide the user's playlist list for the "Add to playlist" submenu.
+        void setUserPlaylists(const QVector<QPair<qint64, QString>> &playlists);
+
     signals:
         void playTrackRequested(qint64 trackId);
+        void addToPlaylistRequested(qint64 trackId, qint64 playlistId);
+        void removeFromPlaylistRequested(qint64 playlistId, qint64 playlistTrackId);
 
     private:
         TrackListModel *m_model   = nullptr;
         QobuzBackend   *m_backend = nullptr;
         PlayQueue      *m_queue   = nullptr;
+        qint64          m_playlistId = 0;
+        QVector<QPair<qint64, QString>> m_userPlaylists;
 
         void onDoubleClicked(const QModelIndex &index);
         void onContextMenu(const QPoint &pos);