feat(checkout): real Stripe Express + Elements on /checkout

- Add @stripe/stripe-js + @stripe/react-stripe-js
- New src/lib/stripe-client.ts: cached loadStripe helper
- New src/actions/billing/retail-payment-intent.ts: server action
  that creates a PaymentIntent with automatic_payment_methods
- New src/components/storefront/StripeExpressCheckout.tsx: embedded
  ExpressCheckoutElement (Apple Pay, Google Pay, Link, PayPal) +
  PaymentElement (card) + hosted-checkout fallback
- /checkout form is now controlled; StripeExpressCheckout reads
  name/email/stop from form state and confirms in-page via
  stripe.confirmPayment
- /checkout/success handles both ?session_id= and ?payment_intent=
  so embedded + hosted flows both land on the same order-creation
  page using the pending_checkout sessionStorage payload
- Export StopInfo from CartContext and select 'time' on the stops
  fetch so the local Stop shape matches the context's StopInfo
- QuickCartSheet express buttons are visual shortcuts only;
  /checkout auto-renders the real Apple Pay / Google Pay buttons

Requires NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY for the embedded path;
if missing the embedded section falls back to the hosted
Stripe Checkout button. npx tsc --noEmit passes.

src/components/storefront/QuickCartSheet.tsx

@@ -312,7 +312,10 @@ function SheetContent({
 
       {/* Footer — sticky CTA + express */}
       <div className="border-t border-stone-900/10 bg-[#F5EFD9] px-6 sm:px-8 pt-5 pb-7 space-y-3">
-        {/* Express checkout row */}
+        {/* Express checkout row — the buttons are visual shortcuts.
+            /checkout auto-renders Stripe's <ExpressCheckoutElement> for
+            Apple Pay / Google Pay / Link + <PaymentElement> for card via
+            StripeExpressCheckout. The `?express=` query is decorative. */}
         <div>
           <p className="font-mono text-[9px] uppercase tracking-[0.22em] text-stone-500 mb-2 text-center">
             Express checkout