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/css/style.css

@@ -281,6 +281,38 @@ button:active {
   color: #0f0;
 }
 
+/* Game Over Overlay */
+#gameover-screen {
+  position: fixed;
+  top: 0;
+  left: 0;
+  width: 100%;
+  height: 100%;
+  background: rgba(0, 0, 0, 0.85);
+  z-index: 999;
+  display: none;
+  justify-content: center;
+  align-items: center;
+}
+
+#gameover-screen.active {
+  display: flex;
+}
+
+#gameover-screen h2 {
+  font-size: 2rem;
+  margin-bottom: 20px;
+}
+
+#gameover-screen h3 {
+  font-size: 1.5rem;
+  margin: 20px 0;
+}
+
+#gameover-screen #final-scores {
+  margin: 30px 0;
+}
+
 /* Garbage row animation */
 @keyframes shake {
   0%, 100% { transform: translateX(0); }