Update notification sound to notification.mp3

Replaces all references to hohoho.mp3 with notification.mp3 for notification sounds across the app. Moves AudioPreloader to layout component and removes duplicate usage. Adds a guard in BroadcastDialog to prevent duplicate sends.

components/notifications/OrderNotifications.tsx

@@ -33,9 +33,9 @@ export default function OrderNotifications() {
   const audioRef = useRef<HTMLAudioElement | null>(null);
 
   useEffect(() => {
-    audioRef.current = new Audio('/hohoho.mp3');
+    audioRef.current = new Audio('/notification.mp3');
     
-    // Fallback if hohoho.mp3 doesn't exist
+    // Fallback if notification.mp3 doesn't exist
     audioRef.current.addEventListener('error', () => {
       audioRef.current = null;
     });