Show game over as overlay on top of game screen

- Make gameover-screen a fixed overlay with semi-transparent background
- Removed 'screen' class from gameover-screen element
- Game scene remains visible behind game over overlay

public/index.html

@@ -38,8 +38,8 @@
       <div id="game-status"></div>
     </div>
 
-    <!-- Game Over Screen -->
-    <div id="gameover-screen" class="screen">
+    <!-- Game Over Screen (Overlay) -->
+    <div id="gameover-screen">
       <h2>GAME OVER</h2>
       <h3 id="winner-display"></h3>
       <div id="final-scores"></div>