feat: full Discord-style presence tracking

- Announce online immediately on app startup
- Idle detection: unavailable after 10 min of no input, online on return
- Tab visibility: unavailable when hidden, online when focused again
- Page close: offline via fetch+keepalive (survives unload without bfcache penalty)
- hidePresence setting: broadcasts offline and stops all tracking
- Added 'Hide Online Status' toggle in General settings

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/pages/client/ClientNonUIFeatures.tsx

@@ -26,6 +26,7 @@ import { getMxIdLocalPart, mxcUrlToHttp } from '../../utils/matrix';
 import { useSelectedRoom } from '../../hooks/router/useSelectedRoom';
 import { useInboxNotificationsSelected } from '../../hooks/router/useInbox';
 import { useMediaAuthentication } from '../../hooks/useMediaAuthentication';
+import { usePresenceUpdater } from '../../hooks/usePresenceUpdater';
 
 function SystemEmojiFeature() {
   const [twitterEmoji] = useSetting(settingsAtom, 'twitterEmoji');
@@ -127,6 +128,11 @@ function InviteNotifications() {
   );
 }
 
+function PresenceUpdater() {
+  usePresenceUpdater();
+  return null;
+}
+
 function MessageNotifications() {
   const audioRef = useRef<HTMLAudioElement>(null);
   const notifRef = useRef<Notification | undefined>(undefined);
@@ -261,6 +267,7 @@ export function ClientNonUIFeatures({ children }: ClientNonUIFeaturesProps) {
       <SystemEmojiFeature />
       <PageZoomFeature />
       <FaviconUpdater />
+      <PresenceUpdater />
       <InviteNotifications />
       <MessageNotifications />
       {children}