fix: resolve notification routing bug, audio playback format, and backend 500 errors

- Fixed audio playback by downloading a valid mp3 file and importing it directly via Vite.

- Fixed the route collision where DELETE /notifications/clear-all was being captured by /notifications/:id.

- The notification badge now automatically clears (optimistic UI update) when the tray is opened.

- The backend no longer throws a 500 error when querying users during impersonation handoffs.

components/Layout.tsx

@@ -11,6 +11,7 @@ import {
   deleteNotification, clearAllNotifications, returnToSuperAdmin
 } from '../services/dataService';
 import { User } from '../types';
+import notificationSound from '../src/assets/audio/notification.mp3';
 
 const SidebarItem = ({ to, icon: Icon, label, collapsed }: { to: string, icon: any, label: string, collapsed: boolean }) => (
   <NavLink
@@ -592,7 +593,7 @@ export const Layout: React.FC<{ children: React.ReactNode }> = ({ children }) =>
       {/* Hidden Audio Player for Notifications */}
       <audio 
         ref={audioRef} 
-        src={`${import.meta.env.PROD ? '' : 'http://localhost:3001'}/audio/notification.mp3`} 
+        src={notificationSound} 
         preload="auto" 
       />
     </div>