Fix cart clearing when navigating to checkout

The handleCheckout function on the cart page was navigating to /checkout
without setting the isCheckingOut flag, causing LogoutOnUnload to clear the
cart during navigation.

Now sets the flag BEFORE navigating so LogoutOnUnload skips clearing the cart.

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

src/app/cart/page.tsx

@@ -17,6 +17,11 @@ export default function CartPage() {
   useEffect(() => setMounted(true), []);
 
   const handleCheckout = () => {
+    // Set flag so LogoutOnUnload doesn't clear cart during navigation to checkout
+    if (typeof window !== 'undefined') {
+      sessionStorage.setItem('isCheckingOut', 'true');
+      console.log('💳 Cart checkout button clicked - set isCheckingOut flag');
+    }
     window.location.href = '/checkout';
   };