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/GameCompletionModal.module.css

@@ -1,26 +1,4 @@
-#game-completion-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-content {
-    background-color: #222;
-    padding: 32px 24px 24px 24px;
-    border-radius: 16px;
-    width: 90vw;
-    max-width: 480px;
-    box-shadow: 0 4px 32px rgba(0,0,0,0.7);
-    position: relative;
-    margin: auto;
-    transform: none;
-}
+/* Only GameCompletionModal-specific styles. Shared modal styles are now in Modal.module.css */
 .final-scores {
     margin: 20px 0;
 }
@@ -60,14 +38,6 @@
     margin: 0;
     color: #fff;
 }
-.modal-footer {
-    display: flex;
-    flex-direction: row;
-    gap: 16px;
-    margin-top: 24px;
-    width: 100%;
-    justify-content: center;
-}
 .btn {
     flex: 1;
     padding: 18px 0;
@@ -89,25 +59,7 @@
 .btn--warning:hover {
     background: #d32f2f;
 }
-.close-button {
-    position: absolute;
-    top: 12px;
-    right: 16px;
-    background: none;
-    border: none;
-    color: #aaa;
-    font-size: 2rem;
-    cursor: pointer;
-    z-index: 2;
-}
-.close-button:hover {
-    color: #fff;
-}
 @media (max-width: 600px) {
-    .modal-content {
-        padding: 16px 4px 16px 4px;
-        max-width: 98vw;
-    }
     .btn {
         font-size: 1rem;
         padding: 14px 0;