FIX: Prevent multiplayer initialization before game is ready

- Add polling mechanism to wait for window.game initialization
- Disable multiplayer button until game is fully initialized
- Enable button in DOMContentLoaded after game creation
- Prevents 'Game instance not found' error on production

This fixes the critical issue where clicking Multiplayer too quickly
would fail because window.game wasn't ready yet.

game.js

@@ -247,9 +247,15 @@ class Connect5Game {
   }
 }
 
-// Initialize game when DOM is loaded
 // Initialize game when DOM is loaded
 window.game = null;
 document.addEventListener("DOMContentLoaded", () => {
   window.game = new Connect5Game();
+  
+  // Enable multiplayer button now that game is ready
+  const multiplayerBtn = document.getElementById('multiplayerModeBtn');
+  if (multiplayerBtn) {
+    multiplayerBtn.disabled = false;
+    console.log('✅ Game initialized, multiplayer button enabled');
+  }
 });