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/backend/qobuzbackend.cpp

@@ -186,6 +186,21 @@ quint64 QobuzBackend::duration() const { return qobuz_backend_get_duration(m_bac
 int     QobuzBackend::volume()   const { return qobuz_backend_get_volume(m_backend); }
 int     QobuzBackend::state()    const { return qobuz_backend_get_state(m_backend); }
 
+quint32 QobuzBackend::vizRead(float *buf, quint32 maxSamples)
+{
+    return qobuz_backend_viz_read(m_backend, buf, maxSamples);
+}
+
+quint32 QobuzBackend::vizSampleRate() const
+{
+    return qobuz_backend_viz_sample_rate(m_backend);
+}
+
+quint32 QobuzBackend::vizChannels() const
+{
+    return qobuz_backend_viz_channels(m_backend);
+}
+
 // ---- private slots ----
 
 void QobuzBackend::onPositionTick()