Add /checkout page

Fixes the dead "Zur Kasse gehen" link on /cart. Built against the actual
Figma frame (page-checkout, node 4761:465, jCCZyh1DGwdjpv1wGge9To):
breadcrumb, 4-step progress stepper (Warenkorb done / Adresse active /
Zahlung+Abschluss upcoming), Rechnungsadresse form, Versandart (real
radio toggle — switching to Express actually updates the shown total),
Zahlungsart (Kreditkarte/PayPal/Überweisung with the real exported
payment-network logos), sidebar order summary reusing the same
cart/products data as /cart, trust bullets, and a newsletter hint box
(divider under the text, not the icon — deliberately different from
/cart's own version per this page's own Figma spec).

No real payment processing — "Jetzt kaufen" links to
/bestellbestaetigung (not yet built) same as the rest of this prototype
site's forms (cart, newsletter signup) don't hit a real backend either.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/checkout/page.tsx

@@ -0,0 +1,26 @@
+import type { Metadata } from "next";
+import { CheckoutContent } from "./components/CheckoutContent";
+import { TrustRow } from "../components/TrustRow";
+import { Footer } from "../components/Footer";
+
+// robots: noindex — transactional page, same reasoning as /cart.
+export const metadata: Metadata = {
+  title: "Checkout",
+  description: "Schließe deine Bestellung bei einfach produktiv ab.",
+  robots: {
+    index: false,
+    follow: true,
+  },
+};
+
+export default function CheckoutPage() {
+  return (
+    <>
+      <main className="flex flex-col flex-1 bg-bg-base">
+        <CheckoutContent />
+        <TrustRow />
+      </main>
+      <Footer />
+    </>
+  );
+}