Build a shopping cart with checkout

• A product list with Add to cart; the cart shows each line with quantity stepper, remove button and line total; quantity cannot go below 1 (use remove for zero) or above a stock limit
• Totals: subtotal, tax (a fixed percentage, for example 8%), discount from a coupon code, and the grand total, all computed in integer cents and displayed with Intl.NumberFormat
• Coupon codes validated against a small table (for example a percentage code and a fixed-amount code); invalid or expired codes show an error; only one coupon at a time; the discount never makes the total negative
• The cart persists in localStorage and is restored on reload, with stale data handled safely (removed products, changed prices)
• A checkout form (name, email, address, card number with Luhn check, expiry, CVV) with inline validation on blur and on submit, disabled submit while invalid, and a pending state
• Submitting calls a mocked order API, shows success with an order summary and clears the cart; shows an error and keeps the cart on failure
• Empty cart state with a link back to the products

Only after every requirement works.

• Cart contents sync between tabs
• Shipping options that change the total
• Optimistic stock check against the mocked API before submitting