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/ui.js

@@ -10,7 +10,6 @@ class UIManager {
     };
 
     this.inputs = {
-      roomName: document.getElementById('room-name'),
       playerName: document.getElementById('player-name')
     };
 
@@ -22,7 +21,6 @@ class UIManager {
     };
 
     this.displays = {
-      lobbyRoomName: document.getElementById('lobby-room-name'),
       gameRoomName: document.getElementById('game-room-name'),
       playerList: document.getElementById('player-list'),
       battleGrid: document.getElementById('battle-grid'),
@@ -41,9 +39,6 @@ class UIManager {
     this.buttons.backToLobby.addEventListener('click', () => this.handleBackToLobby());
 
     // Allow Enter key to submit forms
-    this.inputs.roomName.addEventListener('keypress', (e) => {
-      if (e.key === 'Enter') this.handleJoin();
-    });
     this.inputs.playerName.addEventListener('keypress', (e) => {
       if (e.key === 'Enter') this.handleJoin();
     });
@@ -55,21 +50,19 @@ class UIManager {
   }
 
   handleJoin() {
-    const roomName = this.inputs.roomName.value.trim();
     const playerName = this.inputs.playerName.value.trim();
 
-    if (!roomName || !playerName) {
-      this.showMessage('Please enter room name and your name');
+    if (!playerName) {
+      this.showMessage('Please enter your name');
       return;
     }
 
-    // Emit join event
+    // Set listener for player joined event
     network.setListener('player-joined', () => {
       this.showScreen('lobby');
-      this.displays.lobbyRoomName.textContent = roomName;
     });
 
-    network.joinRoom(roomName, playerName);
+    network.joinLobby(playerName);
   }
 
   handleReady() {
@@ -79,7 +72,7 @@ class UIManager {
   }
 
   handleLeave() {
-    network.leaveRoom();
+    network.leaveLobby();
     this.showScreen('room');
   }