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/pages/EventChatPage.jsx

@@ -246,16 +246,38 @@ const EventChatPage = () => {
 
   // Combine checked-in users with online status
   const getAllDisplayUsers = () => {
-    const activeUserIds = new Set(activeUsers.map(u => u.userId));
+    // Create map of userId -> user data from activeUsers (real-time socket data)
+    const activeUsersMap = new Map(activeUsers.map(u => [u.userId, { ...u, isOnline: true }]));
 
-    // Merge checked-in users with online status
-    const allUsers = checkedInUsers.map(user => ({
-      ...user,
-      isOnline: activeUserIds.has(user.userId),
-    }));
+    // Create map of userId -> user data from checkedInUsers (database data)
+    const checkedInUsersMap = new Map(checkedInUsers.map(u => [u.userId, { ...u, isOnline: false }]));
 
-    // Sort: online first, then offline
-    return allUsers.sort((a, b) => {
+    // Merge: prioritize activeUsers data (it's real-time), add offline users from checkedInUsers
+    const mergedMap = new Map();
+
+    // Add all active users (online)
+    activeUsers.forEach(u => {
+      const checkedInData = checkedInUsersMap.get(u.userId);
+      mergedMap.set(u.userId, {
+        userId: u.userId,
+        username: u.username,
+        avatar: u.avatar,
+        // Use additional data from checkedInUsers if available
+        ...checkedInData,
+        // Override with socket data
+        isOnline: true,
+      });
+    });
+
+    // Add offline checked-in users (not in activeUsers)
+    checkedInUsers.forEach(u => {
+      if (!mergedMap.has(u.userId)) {
+        mergedMap.set(u.userId, { ...u, isOnline: false });
+      }
+    });
+
+    // Convert to array and sort: online first, then offline
+    return Array.from(mergedMap.values()).sort((a, b) => {
       if (a.isOnline === b.isOnline) return 0;
       return a.isOnline ? -1 : 1;
     });