Add to cart is the primary conversion action.
Button states:
- Default: "Add to Cart"
- Loading: Spinner or "Adding..."
- Success: "Added!" with checkmark
- Disabled: When out of stock or no variant selected
After adding:
- Show confirmation (toast or mini cart)
- Update cart icon count
- Option to continue shopping or go to cart
Optimistic update: Update cart immediately, rollback on error.
Accessibility: Announce "Added to cart" for screen readers.