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/app.js

@@ -33,7 +33,7 @@ function setupNetworkListeners() {
   // Game started
   network.setListener('game-started', (players, states) => {
     ui.showScreen('game');
-    ui.displays.gameRoomName.textContent = network.currentRoom;
+    ui.displays.gameRoomName.textContent = 'GLOBAL LOBBY';
 
     // Clear old boards
     renderer.clearAll();