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/styles/index.css

@@ -26,7 +26,7 @@ input, select {
 
 /* Responsive adjustments for fullscreen toggle button */
 @media screen and (max-width: 480px) {
-    .fullscreen-toggle {
+    .fullscreenToggle {
         bottom: 15px;
         right: 15px;
         width: 40px;
@@ -55,6 +55,31 @@ input, select {
     background: #333;
 }
 
+/* Shared utility classes for buttons and layout */
+.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;
+}
+.nav-buttons {
+    display: flex;
+    flex-direction: column;
+    gap: 12px;
+    margin: 16px 0 0 0;
+}
+
 /* Modal overlay (global, not component-specific) */
 .modal {
     position: fixed;