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/modals/broadcast-dialog.tsx

@@ -83,6 +83,11 @@ export default function BroadcastDialog({ open, setOpen }: BroadcastDialogProps)
   };
 
   const sendBroadcast = async () => {
+    // Prevent duplicate sends
+    if (isSending) {
+      return;
+    }
+
     if ((!broadcastMessage || !broadcastMessage.trim()) && !selectedImage) {
       toast.warning("Please provide a message or image to broadcast.");
       return;