fix(checkout): skip loadStripe when no publishable key is set

loadStripe(undefined) threw "Expected publishable key to be of type
string" on every page that imports CheckoutOverlay. Production sets the
key, so live behaviour is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/components/CheckoutOverlay.tsx

@@ -9,7 +9,10 @@ import { PaymentStep } from './checkout/PaymentStep';
 import { ReviewStep } from './checkout/ReviewStep';
 import { ConfirmationStep } from './checkout/ConfirmationStep';
 
-const stripePromise = loadStripe(process.env.NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY!);
+// Without a publishable key (local builds), loadStripe throws on every page that
+// imports this module. Elements accepts null and the payment step stays empty.
+const publishableKey = process.env.NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY;
+const stripePromise = publishableKey ? loadStripe(publishableKey) : null;
 
 const STEPS: { id: CheckoutStep; label: string }[] = [
   { id: 'shipping', label: 'Shipping' },