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

backend/src/socket/index.js

@@ -225,6 +225,7 @@ function initializeSocket(httpServer) {
 
         // Broadcast updated active users list
         const users = Array.from(activeUsers.get(eventId)).map(u => JSON.parse(u));
+        console.log(`📡 Emitting active_users to room ${roomName}:`, users.length, 'users');
         io.to(roomName).emit('active_users', users);
 
         // Notify room about new user
@@ -276,6 +277,7 @@ function initializeSocket(httpServer) {
 
         // Broadcast updated list
         const updatedUsers = Array.from(users).map(u => JSON.parse(u));
+        console.log(`📡 Emitting active_users (after leave) to room ${roomName}:`, updatedUsers.length, 'users');
         io.to(roomName).emit('active_users', updatedUsers);
       }
 
@@ -599,6 +601,7 @@ function initializeSocket(httpServer) {
 
           // Broadcast updated list
           const updatedUsers = Array.from(users).map(u => JSON.parse(u));
+          console.log(`📡 Emitting active_users (after disconnect) to room ${socket.currentEventRoom}:`, updatedUsers.length, 'users');
           io.to(socket.currentEventRoom).emit('active_users', updatedUsers);
 
           // Notify about user leaving