Improve disabled button UX with custom message and no hover effect

- Disable hover animation on disabled add-to-cart button
- Display "Choisissez une option" message when button is disabled
- Add cursor: not-allowed and opacity styling for disabled state
- Format SCSS for better code consistency

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

assets/css/style.css

@@ -125,7 +125,8 @@ main {
   text-decoration: none;
   cursor: pointer;
 }
-.btn__default .icon, .btn__default .txt {
+.btn__default .icon,
+.btn__default .txt {
   z-index: 10;
 }
 .btn__default .icon {
@@ -160,13 +161,17 @@ main {
   z-index: 0;
   transition: width 0.2s;
 }
-.btn__default:hover .txt {
+.btn__default:hover:not(:disabled) .txt {
   color: var(--color-bg);
   display: block;
 }
-.btn__default:hover::after {
+.btn__default:hover:not(:disabled)::after {
   width: 100%;
 }
+.btn__default:disabled {
+  cursor: not-allowed;
+  opacity: 0.6;
+}
 
 #btn--don__mobile {
   width: 100%;