Add unready toggle in lobby

- Server: Handle unready event to set player ready state to false
- Client: Ready button toggles between READY and UNREADY
- Client: Unready sends unready socket event to server

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

public/js/network.js

@@ -110,6 +110,11 @@ class NetworkManager {
     this.socket.emit('ready');
   }
 
+  unready() {
+    if (!this.socket) return;
+    this.socket.emit('unready');
+  }
+
   sendMove(direction) {
     if (!this.socket || !this.currentPlayerId) return;
     this.socket.emit('player-move', { playerId: this.currentPlayerId, direction });