fix: modal overlay and game screen styling

- Move modal overlay CSS to global stylesheet for reliable overlay behavior
- Render GameCompletionModal outside main container for true overlay effect
- Refactor GameCompletionModal to use global overlay and local content styles
- Fix player score layout, color, and button styling on game detail screen
- Add global .modal and .modal.show classes to index.css
- Clean up CSS modules for modal content and responsive design

Fixes #<issue_number>

src/styles/index.css

@@ -76,4 +76,20 @@ input, select {
 
 .new-game-button:hover {
   background: #333;
+}
+
+.modal {
+  position: fixed;
+  top: 0;
+  left: 0;
+  width: 100vw;
+  height: 100vh;
+  background-color: rgba(0, 0, 0, 0.8);
+  z-index: 9999;
+  display: flex;
+  justify-content: center;
+  align-items: center;
+}
+.modal.show {
+  display: flex;
 }