Add debug logging for checkout and LogoutOnUnload

Add console logs to track:
- When isCheckingOut flag is set in checkout button
- When LogoutOnUnload handleUnload is called
- Whether isCheckingOut flag is detected
- When redirecting to Stripe URL

This helps diagnose why cart is still being cleared during checkout.

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

src/app/checkout/client.tsx

@@ -128,6 +128,7 @@ export default function CheckoutPageClient({ isLoggedIn, customer }: CheckoutPag
                     // Mark that we're checking out so LogoutOnUnload doesn't clear cart
                     if (typeof window !== 'undefined') {
                       sessionStorage.setItem('isCheckingOut', 'true');
+                      console.log('Checkout button clicked: isCheckingOut flag set');
                     }
 
                     const res = await fetch('/api/checkout', {
@@ -143,6 +144,7 @@ export default function CheckoutPageClient({ isLoggedIn, customer }: CheckoutPag
                       }
                       throw new Error(data.error ?? 'Something went wrong starting checkout.');
                     }
+                    console.log('Redirecting to Stripe:', data.url);
                     window.location.href = data.url;
                   } catch (err) {
                     const errorMsg = err instanceof Error ? err.message : 'Something went wrong.';