fix: gapless toggle now actually controls audio output lifecycle

When gapless is off, the AudioOutput is dropped after each track ends
naturally, producing a real gap on the next play. When on, the output
stays alive so tracks transition seamlessly. Also re-adds URL prefetch
gating behind the same toggle.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

rust/src/player/mod.rs

@@ -51,6 +51,8 @@ pub struct PlayerStatus {
     pub seek_target_secs: Arc<AtomicU64>,
     /// Linear gain factor to apply (1.0 = unity). Updated each time a new track starts.
     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>,
 }
 
 impl PlayerStatus {
@@ -65,6 +67,7 @@ impl PlayerStatus {
             seek_requested: Arc::new(AtomicBool::new(false)),
             seek_target_secs: Arc::new(AtomicU64::new(0)),
             replaygain_gain: Arc::new(std::sync::Mutex::new(1.0)),
+            gapless: Arc::new(AtomicBool::new(false)),
         }
     }
 
@@ -197,7 +200,10 @@ fn player_loop(rx: std::sync::mpsc::Receiver<PlayerCommand>, status: PlayerStatu
                 pending_info = Some(next_info);
             }
             Ok(None) => {
-                // Track finished naturally
+                // Track finished naturally — tear down audio output if gapless is off
+                if !status.gapless.load(Ordering::Relaxed) {
+                    audio_output = None;
+                }
                 *status.state.lock().unwrap() = PlayerState::Idle;
                 status.track_finished.store(true, Ordering::SeqCst);
             }