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/lib/customerAuth.ts

@@ -674,6 +674,11 @@ export type CustomerOrderDetail = CustomerOrder & {
   correctionInvoiceIssuedAt: string | null;
   carrier: string | null;
   trackingNumber: string | null;
+  // Raw media id, not populated — this fetch stays depth=0 (see this
+  // function's own comment on why), so the order-detail page resolves the
+  // actual download URL itself via a separate media lookup when present.
+  dhlReturnLabelMedia: number | null;
+  dhlReturnTrackingNumber: string | null;
   customerFirstName: string;
   customerLastName: string;
   customerEmail: string;