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/context/CartContext.tsx

@@ -20,6 +20,8 @@ type StopInfo = {
   brand_id: string;
 };
 
+export type { StopInfo };
+
 type CartContextType = {
   cart: CartItem[];
   subtotal: number;
@@ -334,7 +336,7 @@ export function CartProvider({ children }: { children: ReactNode }) {
   );
 }
 
-export function useCart() {
+export function useCart(): CartContextType {
   const context = useContext(CartContext);
   if (!context) throw new Error("useCart must be used inside CartProvider");
   return context;