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/js/ui.js

@@ -72,6 +72,7 @@ class UIManager {
   }
 
   handleBackToLobby() {
+    this.hideGameOver();
     this.showScreen('room');
   }
 
@@ -119,7 +120,11 @@ class UIManager {
       this.displays.finalScores.appendChild(item);
     });
 
-    this.showScreen('gameover');
+    this.screens.gameover.classList.add('active');
+  }
+
+  hideGameOver() {
+    this.screens.gameover.classList.remove('active');
   }
 }