feat(dashboard): add online count for events

Show real-time count of users currently in each event chat room.
- Backend: Export getEventsOnlineCounts from socket module
- Dashboard API: Include onlineCount for each active event
- Frontend: Display online count with animated green dot indicator

backend/src/socket/index.js

@@ -461,4 +461,21 @@ function initializeSocket(httpServer) {
   return io;
 }
 
-module.exports = { initializeSocket, getIO };
+// Get count of online users for a specific event
+function getEventOnlineCount(eventId) {
+  if (!activeUsers.has(eventId)) {
+    return 0;
+  }
+  return activeUsers.get(eventId).size;
+}
+
+// Get online counts for multiple events
+function getEventsOnlineCounts(eventIds) {
+  const counts = {};
+  for (const eventId of eventIds) {
+    counts[eventId] = getEventOnlineCount(eventId);
+  }
+  return counts;
+}
+
+module.exports = { initializeSocket, getIO, getEventOnlineCount, getEventsOnlineCounts };