feat: show top tracks on artist profile with play/shuffle

- Adds extra=topTracks to artist/page API request
- Embeds a List::Tracks widget at the top of ArtistView showing
  the artist's most popular tracks, with Play and Shuffle buttons
- Bubbles playTrackRequested through MainContent up to MainWindow
- Also adds the viz PCM ring buffer FFI infrastructure (for future
  spectrum widget) to the Rust backend

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

src/mainwindow.cpp

@@ -155,6 +155,8 @@ MainWindow::MainWindow(QobuzBackend *backend, QWidget *parent)
             this, &MainWindow::onSearchAlbumSelected);
     connect(m_content, &MainContent::artistRequested,
             this, &MainWindow::onSearchArtistSelected);
+    connect(m_content, &MainContent::playTrackRequested,
+            this, &MainWindow::onPlayTrackRequested);
 
     // ---- Queue panel ----
     connect(m_queuePanel, &QueuePanel::skipToTrackRequested,