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.hpp

@@ -68,6 +68,11 @@ public:
     /// 1 = playing, 2 = paused, 0 = idle
     int     state() const;
 
+    // --- visualizer PCM ---
+    quint32 vizRead(float *buf, quint32 maxSamples);
+    quint32 vizSampleRate() const;
+    quint32 vizChannels() const;
+
 signals:
     // auth
     void loginSuccess(const QString &token, const QJsonObject &user);