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/GameList.module.css

@@ -1,3 +1,4 @@
+/* GameList-specific styles only. Shared utility classes are now in global CSS. */
 .screen.active {
     display: block;
     opacity: 1;
@@ -21,23 +22,6 @@
     flex: 1;
     overflow-y: auto;
 }
-.nav-buttons {
-    display: flex;
-    flex-direction: column;
-    gap: 10px;
-    margin: 20px 0 40px 0;
-}
-.btn {
-    flex: 1;
-    min-width: 100px;
-    padding: 20px;
-    color: white;
-    border: none;
-    border-radius: 0;
-    font-size: 20px;
-    cursor: pointer;
-    touch-action: manipulation;
-}
 .filter-buttons {
     display: flex;
     gap: 8px;