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.

lib/notification-context.tsx

@@ -92,7 +92,7 @@ export function NotificationProvider({ children }: NotificationProviderProps) {
     }
 
     // Initialize audio
-    audioRef.current = new Audio('/hohoho.mp3');
+    audioRef.current = new Audio('/notification.mp3');
     audioRef.current.addEventListener('error', () => {
       audioRef.current = null;
     });