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/mod.rs

@@ -1,14 +1,18 @@
 mod decoder;
 pub mod output;
 
+use rb::{SpscRb, RB};
 use std::sync::{
-    atomic::{AtomicBool, AtomicU64, AtomicU8, Ordering},
+    atomic::{AtomicBool, AtomicU32, AtomicU64, AtomicU8, Ordering},
     Arc,
 };
 use std::time::Duration;
 
 use crate::api::TrackDto;
 
+/// Size of the visualizer ring buffer in f32 samples (~180ms at 44.1kHz stereo).
+const VIZ_RING_SIZE: usize = 16 * 1024;
+
 #[derive(Debug, Clone)]
 pub enum PlayerCommand {
     Play(TrackInfo),
@@ -50,10 +54,17 @@ pub struct PlayerStatus {
     pub replaygain_gain: Arc<std::sync::Mutex<f32>>,
     /// When false the audio output is torn down after each track, producing a gap.
     pub gapless: Arc<AtomicBool>,
+    /// Visualizer ring buffer (consumer side, read by FFI).
+    pub viz_ring: Arc<SpscRb<f32>>,
+    pub viz_consumer: Arc<std::sync::Mutex<rb::Consumer<f32>>>,
+    pub viz_sample_rate: Arc<AtomicU32>,
+    pub viz_channels: Arc<AtomicU32>,
 }
 
 impl PlayerStatus {
     pub fn new() -> Self {
+        let viz_ring = Arc::new(SpscRb::new(VIZ_RING_SIZE));
+        let viz_consumer = Arc::new(std::sync::Mutex::new(viz_ring.consumer()));
         Self {
             state: Arc::new(std::sync::Mutex::new(PlayerState::Idle)),
             position_secs: Arc::new(AtomicU64::new(0)),
@@ -65,6 +76,10 @@ impl PlayerStatus {
             seek_target_secs: Arc::new(AtomicU64::new(0)),
             replaygain_gain: Arc::new(std::sync::Mutex::new(1.0)),
             gapless: Arc::new(AtomicBool::new(false)),
+            viz_ring,
+            viz_consumer,
+            viz_sample_rate: Arc::new(AtomicU32::new(0)),
+            viz_channels: Arc::new(AtomicU32::new(0)),
         }
     }