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/mainwindow.hpp

@@ -13,6 +13,9 @@
 #include <QMainWindow>
 #include <QDockWidget>
 #include <QJsonObject>
+#include <QVector>
+#include <QPair>
+#include <QString>
 
 class MainWindow : public QMainWindow
 {
@@ -38,6 +41,8 @@ private slots:
     void onSearchAlbumSelected(const QString &albumId);
     void onSearchArtistSelected(qint64 artistId);
     void onSearchToggled(bool visible);
+    void onPlaylistCreated(const QJsonObject &playlist);
+    void onUserPlaylistsChanged(const QVector<QPair<qint64, QString>> &playlists);
 
     void showLoginDialog();
     void showSettingsDialog();
@@ -45,6 +50,7 @@ private slots:
 private:
     QobuzBackend    *m_backend     = nullptr;
     PlayQueue       *m_queue       = nullptr;
+    QVector<QPair<qint64, QString>> m_userPlaylists;
     MainToolBar     *m_toolBar     = nullptr;
     MainContent     *m_content     = nullptr;
     List::Library   *m_library     = nullptr;