fix: iOS push notification prompt

iOS requires user gesture for Notification.requestPermission().
- Added NotificationPrompt banner that shows on permission='default'
- autoSubscribePush only fires when already 'granted' (re-subscribe on restore)
- Banner uses tap handler for permission request (works on iOS)
- Gruvbox themed, dismissible, bottom-center toast

web/src/components/Layout.tsx

@@ -8,6 +8,7 @@ import { useLayoutStore } from '../stores/layout.ts';
 import { ServerBar } from './ServerBar.tsx';
 import { ChannelList } from './ChannelList.tsx';
 import { ConversationList } from './ConversationList.tsx';
+import { NotificationPrompt } from './NotificationPrompt.tsx';
 import { MemberList } from './MemberList.tsx';
 import { VoicePanel } from './VoicePanel.tsx';
 import { ServerSettingsModal } from './ServerSettingsModal.tsx';
@@ -223,6 +224,7 @@ export function Layout() {
           <span>{new Date().toISOString().slice(0, 10)}</span>
         </div>
       </div>
+      <NotificationPrompt />
     </div>
   );
 }