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/lib.rs

@@ -478,6 +478,11 @@ pub unsafe extern "C" fn qobuz_backend_set_replaygain(ptr: *mut Backend, enabled
     (*ptr).0.replaygain_enabled.store(enabled, std::sync::atomic::Ordering::Relaxed);
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn qobuz_backend_set_gapless(ptr: *mut Backend, enabled: bool) {
+    (*ptr).0.player.status.gapless.store(enabled, std::sync::atomic::Ordering::Relaxed);
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn qobuz_backend_prefetch_track(
     ptr: *mut Backend,