Ads play once on channel 11, separate from music

- Add dedicated ad playback system (no loop, own channel)
- Ad channel defaults to 11, saved/loaded with audio settings
- Separate play_ad/stop_ad methods and API endpoints
- Frontend stop button now calls /api/ads/stop instead of stopMusic

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

frontend/js/app.js

@@ -131,7 +131,7 @@ function initEventListeners() {
 
     // Ads
     document.getElementById('ad-play-btn')?.addEventListener('click', playAd);
-    document.getElementById('ad-stop-btn')?.addEventListener('click', stopMusic);
+    document.getElementById('ad-stop-btn')?.addEventListener('click', stopAd);
 
     // Settings
     document.getElementById('settings-btn')?.addEventListener('click', async () => {
@@ -686,6 +686,10 @@ async function playAd() {
     });
 }
 
+async function stopAd() {
+    await fetch('/api/ads/stop', { method: 'POST' });
+}
+
 
 // --- Sound Effects (Server-Side) ---
 async function loadSounds() {