feat(realtime): live online/offline events and unified search

- add websocket events user_online/user_offline
- broadcast presence changes on first connect and final disconnect only
- apply live presence updates in web chat store and realtime hook
- move public discover into unified left search (users + groups/channels)
- remove separate Discover Chats dialog/menu entry

app/realtime/schemas.py

@@ -15,6 +15,8 @@ RealtimeEventName = Literal[
     "typing_stop",
     "message_read",
     "message_delivered",
+    "user_online",
+    "user_offline",
     "error",
 ]