refactor: deduplicate modal/button styles and enforce global utility usage

- Consolidated all modal-related styles into Modal.module.css; ValidationModal.module.css is now deprecated
- All main action/navigation buttons in NewGame and GameDetail use global .btn/.nav-buttons utility classes
- Removed duplicate utility classes from component CSS files
- Fixed .fullscreenToggle class naming for consistency
- Cleaned up component CSS to only contain component-specific styles
- Updated GameCompletionModal to use shared modal styles

This ensures DRY, maintainable, and consistent styling across the app.

src/components/Modal.module.css

@@ -1,3 +1,4 @@
+/* Consolidated modal styles for all modals */
 .modal {
     display: none;
     position: fixed;
@@ -31,6 +32,8 @@
     font-size: 24px;
     cursor: pointer;
     color: #888;
+    background: none;
+    border: none;
 }
 .close-button:hover {
     color: white;