🎨 Palette: Improve Button loading state accessibility

- Add `aria-busy` attribute to Button when loading.
- Refactor Button rendering to keep children in DOM (visually hidden) instead of unmounting.
- Fix layout shift regression by replicating flex properties in content wrapper.
- Move inline styles to CSS modules.
- Add tests for loading state accessibility.

Co-authored-by: ragusa-it <196988693+ragusa-it@users.noreply.github.com>

src/components/ui/Button.tsx

@@ -24,14 +24,18 @@ export function Button({
       type={type}
       className={`${styles.button} ${styles[variant]} ${styles[size]} ${className || ''}`}
       disabled={disabled || isLoading}
+      aria-busy={isLoading}
       whileHover={{ scale: 1.02 }}
       whileTap={{ scale: 0.98 }}
       {...props}
     >
-      {isLoading ? (
-        <span className={styles.loader} />
-      ) : (
-        children
+      <span className={`${styles.content} ${isLoading ? styles.contentHidden : ''}`}>
+        {children}
+      </span>
+      {isLoading && (
+        <span className={styles.loaderWrapper} aria-hidden="true">
+          <span className={styles.loader} />
+        </span>
       )}
     </motion.button>
   );