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

frontend/src/pages/DashboardPage.jsx

@@ -326,6 +326,12 @@ const EventCard = ({ event }) => {
         <div className="flex items-center gap-2">
           <Users className="w-4 h-4 flex-shrink-0" />
           <span>{event.participantsCount} participants</span>
+          {event.onlineCount > 0 && (
+            <span className="text-green-600 flex items-center gap-1">
+              <span className="w-2 h-2 bg-green-500 rounded-full animate-pulse" />
+              {event.onlineCount} online
+            </span>
+          )}
         </div>
       </div>