feat: queue panel skip-to-track and drag reorder; remove visualizer

Queue panel:
- Double-clicking an upcoming track skips to it immediately: drops all
  tracks before it from the queue and starts playback (skipToUpcoming)
- Items can be dragged to reorder; rowsMoved rebuilds the queue via
  setUpcomingOrder()
- Track JSON stored per-item so order survives drag operations
- New PlayQueue methods: skipToUpcoming(), setUpcomingOrder()
- New QueuePanel signal: skipToTrackRequested(qint64) wired to MainWindow

Remove visualizer:
- Drop VisualizerWidget, Qt6::OpenGLWidgets, projectM CMake detection
- Remove qobuz_backend_read_pcm FFI (Rust + C header + Qt wrapper)
- Remove pcm_visualizer from PlayerStatus and PCM tap from AudioOutput

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

src/mainwindow.hpp

@@ -52,7 +52,7 @@ private:
     QueuePanel      *m_queuePanel  = nullptr;
     SidePanel::View *m_sidePanel   = nullptr;
     QDockWidget     *m_libraryDock = nullptr;
-    LastFmScrobbler *m_scrobbler   = nullptr;
+    LastFmScrobbler  *m_scrobbler   = nullptr;
 
     void setupMenuBar();
     void tryRestoreSession();