Button
Copied from Academia `button.tsx` — CVA variants, sizes, and `isLoading` (spinner overlap, no layout shift).
Loading (isLoading)
Same Academia API: pass isLoading on Button. Spinner replaces the label without changing width.
Copied from Academia `button.tsx` — CVA variants, sizes, and `isLoading` (spinner overlap, no layout shift).
Same Academia API: pass isLoading on Button. Spinner replaces the label without changing width.