Fix cart clearing during Stripe checkout redirect

Use sessionStorage flag to prevent LogoutOnUnload from clearing cart and logging
out when navigating to Stripe checkout.

Changes:
- Checkout button sets 'isCheckingOut' flag in sessionStorage before API call
- LogoutOnUnload checks this flag and skips logout if checkout in progress
- Flag is removed if API returns an error
- Flag persists during navigation to Stripe URL

This ensures cart and login persist through Stripe payment flow.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/app/checkout/client.tsx

@@ -117,6 +117,9 @@ export default function CheckoutPageClient({ isLoggedIn, customer }: CheckoutPag
                   setCheckingOut(true);
                   setCheckoutError(null);
                   try {
+                    // Mark that we're checking out so LogoutOnUnload doesn't clear cart
+                    sessionStorage.setItem('isCheckingOut', 'true');
+
                     const res = await fetch('/api/checkout', {
                       method: 'POST',
                       headers: { 'Content-Type': 'application/json' },
@@ -125,6 +128,7 @@ export default function CheckoutPageClient({ isLoggedIn, customer }: CheckoutPag
                     const data = await res.json();
                     console.log('Checkout response:', { status: res.status, ok: res.ok, data });
                     if (!res.ok || !data.url) {
+                      sessionStorage.removeItem('isCheckingOut');
                       throw new Error(data.error ?? 'Something went wrong starting checkout.');
                     }
                     window.location.href = data.url;