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>

rust/src/lib.rs

@@ -590,6 +590,34 @@ pub unsafe extern "C" fn qobuz_backend_get_user(ptr: *mut Backend) {
     });
 }
 
+// ---------- Visualizer PCM access ----------
+
+/// Read up to `max_samples` f32 PCM values into `buf`.
+/// Returns the number of samples actually read.
+#[no_mangle]
+pub unsafe extern "C" fn qobuz_backend_viz_read(
+    ptr: *mut Backend,
+    buf: *mut f32,
+    max_samples: u32,
+) -> u32 {
+    let consumer = &(*ptr).0.player.status.viz_consumer;
+    let Ok(mut lock) = consumer.try_lock() else { return 0 };
+    let slice = std::slice::from_raw_parts_mut(buf, max_samples as usize);
+    rb::RbConsumer::read(&mut *lock, slice).unwrap_or(0) as u32
+}
+
+/// Returns current sample rate of the audio stream (0 if idle).
+#[no_mangle]
+pub unsafe extern "C" fn qobuz_backend_viz_sample_rate(ptr: *const Backend) -> u32 {
+    (*ptr).0.player.status.viz_sample_rate.load(std::sync::atomic::Ordering::Relaxed)
+}
+
+/// Returns current channel count (0 if idle).
+#[no_mangle]
+pub unsafe extern "C" fn qobuz_backend_viz_channels(ptr: *const Backend) -> u32 {
+    (*ptr).0.player.status.viz_channels.load(std::sync::atomic::Ordering::Relaxed)
+}
+
 // ---------- Playlist management ----------
 
 pub const EV_PLAYLIST_CREATED: c_int = 20;