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.hpp

@@ -36,6 +36,16 @@ public:
     void getFavArtists(quint32 offset = 0, quint32 limit = 200);
     void getUserPlaylists(quint32 offset = 0, quint32 limit = 200);
 
+    // --- playback options ---
+    void setReplayGain(bool enabled);
+    void prefetchTrack(qint64 trackId, int formatId = 6);
+
+    // --- playlist management ---
+    void createPlaylist(const QString &name);
+    void deletePlaylist(qint64 playlistId);
+    void addTrackToPlaylist(qint64 playlistId, qint64 trackId);
+    void deleteTrackFromPlaylist(qint64 playlistId, qint64 playlistTrackId);
+
     // --- fav modification ---
     void addFavTrack(qint64 trackId);
     void removeFavTrack(qint64 trackId);
@@ -66,6 +76,8 @@ signals:
     void albumLoaded(const QJsonObject &album);
     void artistLoaded(const QJsonObject &artist);
     void playlistLoaded(const QJsonObject &playlist);
+    void playlistCreated(const QJsonObject &playlist);
+    void playlistDeleted(const QJsonObject &result);
 
     // favorites
     void favTracksLoaded(const QJsonObject &result);