fix layout and demo payment

web/app/cart/page.js

@@ -57,6 +57,11 @@ function PaymentForm({ clientSecret, totalAmount, onSuccess, onCancel }) {
       <p className="cart-payment-total">
         Total to pay: <strong>${parseFloat(totalAmount).toFixed(2)}</strong>
       </p>
+      <div className="cart-demo-banner">
+        <strong>Demo mode</strong> — no real charge. Use test card:
+        <span className="cart-demo-card">4242 4242 4242 4242</span>
+        · any future date · any 3-digit CVC
+      </div>
       <PaymentElement />
       {payError && <p className="cart-error-msg">{payError}</p>}
       <div className="cart-payment-actions">