Fix lobby: only show players who have joined

Changes:
- Added socket.join(LOBBY_ROOM) when player joins lobby
- Changed io.emit() to io.to(LOBBY_ROOM).emit() for all lobby events
- Players only see lobby events after they click JOIN LOBBY
- Fixed player list to update correctly when players join/ready
- Game now starts properly when all players are ready

Files modified:
- server/index.js: Use Socket.io rooms for lobby scoping
- public/js/app.js: Show lobby screen on player-joined event
- public/js/ui.js: Removed duplicate listener override

public/js/ui.js

@@ -57,11 +57,6 @@ class UIManager {
       return;
     }
 
-    // Set listener for player joined event
-    network.setListener('player-joined', () => {
-      this.showScreen('lobby');
-    });
-
     network.joinLobby(playerName);
   }