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/ChatDetail.tsx

@@ -130,9 +130,9 @@ export default function ChatDetail({ chatId }: { chatId: string }) {
   // Initialize audio element
   useEffect(() => {
     // Create audio element for notification sound
-    audioRef.current = new Audio('/hohoho.mp3');
+    audioRef.current = new Audio('/notification.mp3');
     
-    // Fallback if hohoho.mp3 doesn't exist - use browser API for a simple beep
+    // Fallback if notification.mp3 doesn't exist - use browser API for a simple beep
     audioRef.current.addEventListener('error', () => {
       audioRef.current = null;
     });

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;