Refactor to single global lobby

Changes:
- Removed room-based architecture, now using single global lobby
- Players only need to enter their name to join
- Game starts when all players in lobby are ready (min 2, max 8)
- Simplified UI - no room name field, shows "Global Lobby" header
- Updated all server events to use io.emit instead of io.to(roomName)

Files modified:
- server/index.js: Replaced rooms Map with single lobby object
- public/index.html: Removed room name input, updated button text
- public/js/network.js: Renamed joinRoom/leaveRoom to joinLobby/leaveLobby
- public/js/ui.js: Simplified join flow, removed room name validation
- public/js/app.js: Updated game header to show "GLOBAL LOBBY"
- PLAN.md: Marked all phases as complete

public/js/network.js

@@ -3,7 +3,6 @@
 class NetworkManager {
   constructor() {
     this.socket = null;
-    this.currentRoom = null;
     this.currentPlayerId = null;
     this.players = {};
     this.gameState = {};
@@ -26,7 +25,6 @@ class NetworkManager {
 
     this.socket.on('disconnect', () => {
       console.log('Disconnected from server');
-      this.currentRoom = null;
     });
 
     this.socket.on('player-joined', ({ player, players }) => {
@@ -66,20 +64,13 @@ class NetworkManager {
     });
   }
 
-  joinRoom(roomName, playerName) {
+  joinLobby(playerName) {
     if (!this.socket) return;
-
-    this.currentRoom = roomName;
-    this.socket.emit('join-room', { roomName, playerName });
+    this.socket.emit('join-lobby', { playerName });
   }
 
-  leaveRoom() {
+  leaveLobby() {
     if (!this.socket) return;
-
-    if (this.currentRoom) {
-      this.socket.leave(this.currentRoom);
-    }
-    this.currentRoom = null;
   }
 
   ready() {