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.cpp

@@ -120,6 +120,10 @@ MainWindow::MainWindow(QobuzBackend *backend, QWidget *parent)
     connect(m_content, &MainContent::artistRequested,
             this, &MainWindow::onSearchArtistSelected);
 
+    // ---- Queue panel ----
+    connect(m_queuePanel, &QueuePanel::skipToTrackRequested,
+            this, &MainWindow::onPlayTrackRequested);
+
     // ---- Toolbar toggles ----
     connect(m_toolBar, &MainToolBar::searchToggled, this, &MainWindow::onSearchToggled);
     connect(m_toolBar, &MainToolBar::queueToggled,