Billing address can match shipping without re-entry

Most shoppers pay with a card registered to the same address they're shipping to. Making them type the exact same seven fields twice is pure friction with no payoff. A default-checked "Same as shipping" toggle collapses the billing section to a single checkbox for the common case while preserving separate-address entry for the minority that needs it. Missing this pattern adds roughly thirty seconds of typing and a fresh set of typo opportunities on every checkout.

Add a default-checked "Same as shipping" toggle in src/components/CheckoutAddresses.tsx. When checked, copy shipping values into billing state and hide the billing fields. When unchecked, render the full billing form.

const [sameAsShipping, setSameAsShipping] = useState(true)
<input type="checkbox" checked={sameAsShipping} onChange={e => { setSameAsShipping(e.target.checked); if (e.target.checked) setBillingAddress(shippingAddress) }} />
{!sameAsShipping && <BillingForm value={billingAddress} onChange={setBillingAddress} />}