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

web/src/hooks/useRealtime.ts

@@ -127,6 +127,19 @@ export function useRealtime() {
             chatStore.setMessageDeliveryStatusUpTo(chatId, maxId, "read", authStore.me?.id ?? -1);
           }
         }
+        if (event.event === "user_online" || event.event === "user_offline") {
+          const chatId = Number(event.payload.chat_id);
+          const userId = Number(event.payload.user_id);
+          const isOnline = Boolean(event.payload.is_online);
+          const lastSeenAtRaw = event.payload.last_seen_at;
+          const lastSeenAt = typeof lastSeenAtRaw === "string" ? lastSeenAtRaw : undefined;
+          if (!Number.isFinite(chatId) || !Number.isFinite(userId)) {
+            return;
+          }
+          if (userId !== authStore.me?.id) {
+            chatStore.applyPresenceEvent(chatId, userId, isOnline, lastSeenAt);
+          }
+        }
       };
 
       ws.onclose = () => {