Add DHL checkout integrations (autocomplete, postnummer, return label)

Wires the new backend DHL endpoints into checkout: an address-autocomplete
dropdown on the street fields, live Postnummer validation for Packstation
delivery, and a return-label download link on the order-detail page.
Proxied through Next.js API routes since DHL credentials are tenant-
specific and CheckoutContent is a Client Component.

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

app/konto/bestellungen/[orderNumber]/page.tsx

@@ -7,7 +7,7 @@ import { Footer } from "../../../components/Footer";
 import { VatBreakdown } from "../../../components/VatBreakdown";
 import { formatPrice, formatDate } from "../../../lib/format";
 import { getSessionCustomer, getCustomerOrderDetail, customerOrderAction } from "../../../lib/customerAuth";
-import { getProductImagesByIds, getPaymentMethods, groupPaymentMethodsForCheckout } from "../../../lib/payload";
+import { getProductImagesByIds, getPaymentMethods, groupPaymentMethodsForCheckout, getMediaUrlById } from "../../../lib/payload";
 import { computeTaxBreakdown } from "@einfach-produktiv/invoicing";
 import { buildTrackingUrl, CARRIER_LABELS } from "../../../lib/tracking";
 import { OrderActionButton } from "./components/OrderActionButton";
@@ -48,6 +48,7 @@ export default async function KontoBestellungDetailPage({ params }: { params: Pr
       : order.shippingStreet;
   const action = customerOrderAction(order.status);
   const imagesByProductId = await getProductImagesByIds(order.items.map((item) => item.product));
+  const dhlReturnLabel = order.dhlReturnLabelMedia ? await getMediaUrlById(order.dhlReturnLabelMedia) : null;
   const taxBreakdown = computeTaxBreakdown(order.items, order.subtotal, order.discountAmount, order.shippingCost);
   // Same "Online-Zahlung" grouping/eligibility the switch-to-stripe route
   // itself re-checks authoritatively — only offer the button when it
@@ -105,6 +106,15 @@ export default async function KontoBestellungDetailPage({ params }: { params: Pr
             </div>
           )}
 
+          {dhlReturnLabel && (
+            <div className="flex flex-col gap-1 w-full">
+              <p className="text-label text-text-muted">DHL-Retourenschein{order.dhlReturnTrackingNumber ? ` (${order.dhlReturnTrackingNumber})` : ""}</p>
+              <a href={dhlReturnLabel.url} target="_blank" rel="noopener noreferrer" className="text-body-sm text-brand hover:underline">
+                Retourenschein herunterladen
+              </a>
+            </div>
+          )}
+
           <div className="flex flex-col gap-1 w-full">
             {/* Labeled "Rechnungsadresse" only once there's an actual
                 second (shipping) address to distinguish it from — the