fix(chat): real-time active users list updates

Fixed issue where active users list in event chat did not update in
real-time when new users joined. Users had to refresh the page to see
newly joined participants.

Root Cause:
- getAllDisplayUsers() used checkedInUsers (loaded once from API) as
  base list, with activeUsers (Socket.IO real-time) only for isOnline flag
- When new user joined chat, they appeared in activeUsers but not in
  checkedInUsers, so they were not displayed

Solution:
- Rewrote getAllDisplayUsers() to prioritize activeUsers (real-time data)
- Merges activeUsers (online) with checkedInUsers (offline checked-in users)
- Uses Socket.IO data as source of truth for online users
- Enriches with database data when available (firstName, lastName, etc)
- Sorts online users first, offline second

Changes:
- EventChatPage.jsx: Rewrote getAllDisplayUsers() to merge activeUsers
  with checkedInUsers, prioritizing real-time Socket.IO data
- useEventChat.js: Added debug logging for active_users events
- socket/index.js: Added debug logging for active_users emissions

Testing:
- User A in chat sees User B appear immediately when B joins
- No page refresh required
- Online/offline status updates in real-time

frontend/src/hooks/useEventChat.js

@@ -143,12 +143,14 @@ const useEventChat = (slug, userId, event, messagesContainerRef) => {
 
     // Receive active users list
     socket.on('active_users', (users) => {
+      console.log('📡 Received active_users event:', users);
       // Filter out duplicates and current user
       const uniqueUsers = users
         .filter((u, index, self) =>
           index === self.findIndex((t) => t.userId === u.userId)
         )
         .filter((u) => u.userId !== userId);
+      console.log('👥 Updating active users:', uniqueUsers);
       setActiveUsers(uniqueUsers);
     });