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/shippingDhl.ts

@@ -0,0 +1,46 @@
+// Thin client for the backend's DHL custom endpoints (src/lib/endpoints/
+// dhlValidatePostNumber.ts, dhlAutocompleteAddress.ts) — own copy per repo,
+// same "no shared package yet" convention as app/lib/tracking.ts.
+const PAYLOAD_URL = process.env.PAYLOAD_URL || "https://payload.mk360.de";
+const TENANT_SLUG = "einfach-produktiv";
+
+export async function validateDhlPostNumber(args: {
+  postNumber: string;
+  firstName: string;
+  lastName: string;
+}): Promise<{ ok: true; valid: boolean } | { ok: false; reason: string }> {
+  const params = new URLSearchParams({ tenantSlug: TENANT_SLUG, ...args });
+  try {
+    const res = await fetch(`${PAYLOAD_URL}/api/dhl/validate-post-number?${params}`, {
+      signal: AbortSignal.timeout(8000),
+    });
+    const data = await res.json();
+    if (!res.ok || !data.ok) return { ok: false, reason: data.reason ?? "Postnummer konnte nicht geprüft werden." };
+    return { ok: true, valid: Boolean(data.valid) };
+  } catch {
+    return { ok: false, reason: "Postnummer-Prüfung ist gerade nicht erreichbar." };
+  }
+}
+
+export type DhlAddressSuggestion = {
+  street: string;
+  houseNumber?: string;
+  zip: string;
+  city: string;
+  country: string;
+};
+
+export async function autocompleteDhlAddress(query: string): Promise<DhlAddressSuggestion[]> {
+  if (query.trim().length < 3) return [];
+  const params = new URLSearchParams({ tenantSlug: TENANT_SLUG, query });
+  try {
+    const res = await fetch(`${PAYLOAD_URL}/api/dhl/autocomplete-address?${params}`, {
+      signal: AbortSignal.timeout(5000),
+    });
+    const data = await res.json();
+    if (!res.ok || !data.ok) return [];
+    return data.suggestions as DhlAddressSuggestion[];
+  } catch {
+    return [];
+  }
+}