Add-to-Cart button that fires the cart action for a given product (with selected quantity/color/size) and shows loading/success/error icon states — an inline control, not a standalone page section.

• src/components/registry/blocks/add-to-cart-button.tsx→components/registry/blocks/add-to-cart-button.tsx
• src/hooks/registry/use-cart-action.ts→hooks/registry/use-cart-action.ts