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/NewGame.module.css

@@ -1,3 +1,4 @@
+/* NewGame-specific styles only. Shared utility classes are now in global CSS. */
 .screen {
     position: absolute;
     top: 0;
@@ -97,29 +98,6 @@
     font-size: 1.1rem;
     text-align: center;
 }
-.nav-buttons {
-    display: flex;
-    flex-direction: column;
-    gap: 12px;
-    margin: 16px 0 0 0;
-}
-.btn {
-    flex: 1;
-    min-width: 100px;
-    padding: 18px;
-    color: white;
-    background: #333;
-    border: none;
-    border-radius: 6px;
-    font-size: 1.2rem;
-    font-weight: 600;
-    cursor: pointer;
-    touch-action: manipulation;
-    transition: background 0.2s, color 0.2s;
-}
-.btn:hover {
-    background: #444;
-}
 .new-game-form {
     width: 100%;
     max-width: 700px;