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/player/output.rs

@@ -14,6 +14,7 @@ const RING_BUFFER_SIZE: usize = 32 * 1024;
 
 pub struct AudioOutput {
     ring_buf_producer: rb::Producer<f32>,
+    viz_producer: Option<rb::Producer<f32>>,
     _stream: cpal::Stream,
     pub sample_rate: u32,
     pub channels: usize,
@@ -51,12 +52,17 @@ impl AudioOutput {
 
         Ok(Self {
             ring_buf_producer: producer,
+            viz_producer: None,
             _stream: stream,
             sample_rate,
             channels,
         })
     }
 
+    pub fn set_viz_producer(&mut self, producer: rb::Producer<f32>) {
+        self.viz_producer = Some(producer);
+    }
+
     pub fn write(
         &mut self,
         decoded: AudioBufferRef<'_>,
@@ -70,6 +76,11 @@ impl AudioOutput {
         sample_buf.copy_interleaved_ref(decoded);
         let samples: Vec<f32> = sample_buf.samples().iter().map(|s| s * volume).collect();
 
+        // Best-effort copy for visualizer (non-blocking, ok to drop samples)
+        if let Some(ref mut viz) = self.viz_producer {
+            let _ = viz.write(&samples);
+        }
+
         let mut remaining = &samples[..];
         while !remaining.is_empty() {
             if stop.load(Ordering::SeqCst) {