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/dashboard/ChatTable.tsx

@@ -81,7 +81,7 @@ export default function ChatTable() {
   
   // Initialize audio element for notifications
   useEffect(() => {
-    audioRef.current = new Audio('/hohoho.mp3');
+    audioRef.current = new Audio('/notification.mp3');
     return () => {
       if (audioRef.current) {
         audioRef.current = null;