fix: guard Notification API for unsupported browsers

Check typeof Notification before accessing .permission or
requestPermission(). Fixes crash on browsers/contexts where
the Notification API is unavailable (e.g. some WebViews).

src/App.tsx

@@ -16,7 +16,7 @@ export default function App() {
   } = useGateway();
   const [sidebarOpen, setSidebarOpen] = useState(false);
   const [shortcutsOpen, setShortcutsOpen] = useState(false);
-  const { notify } = useNotifications();
+  const { notify, soundEnabled, toggleSound } = useNotifications();
   const prevMessageCountRef = useRef(messages.length);
 
   // Notify on new assistant messages when tab is not focused
@@ -75,7 +75,7 @@ export default function App() {
         onClose={() => setSidebarOpen(false)}
       />
       <div className="flex-1 flex flex-col min-w-0" aria-hidden={sidebarOpen ? true : undefined}>
-        <Header status={status} sessionKey={activeSession} onToggleSidebar={() => setSidebarOpen(!sidebarOpen)} activeSessionData={sessions.find(s => s.key === activeSession)} onLogout={logout} />
+        <Header status={status} sessionKey={activeSession} onToggleSidebar={() => setSidebarOpen(!sidebarOpen)} activeSessionData={sessions.find(s => s.key === activeSession)} onLogout={logout} soundEnabled={soundEnabled} onToggleSound={toggleSound} />
         <ConnectionBanner status={status} />
         <Chat messages={messages} isGenerating={isGenerating} isLoadingHistory={isLoadingHistory} status={status} onSend={sendMessage} onAbort={abort} />
       </div>