Add isCheckingOut flag to guest checkout flow

Guest checkout handler was missing the sessionStorage flag that prevents
LogoutOnUnload from clearing cart during Stripe redirect. Adds flag before
API call and removes on error.

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

src/app/checkout/client.tsx

@@ -55,6 +55,9 @@ 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' },
@@ -70,6 +73,7 @@ export default function CheckoutPageClient({ isLoggedIn, customer }: CheckoutPag
       });
       const data = await res.json();
       if (!res.ok || !data.url) {
+        sessionStorage.removeItem('isCheckingOut');
         throw new Error(data.error ?? 'Something went wrong starting checkout.');
       }
       window.location.href = data.url;