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/audio-preloader.tsx

@@ -8,7 +8,7 @@ import { useEffect } from "react"
 export function AudioPreloader() {
   useEffect(() => {
     // Preload the audio file
-    const audio = new Audio('/hohoho.mp3')
+    const audio = new Audio('/notification.mp3')
     audio.preload = 'auto'
     
     // Try to load it immediately