Replace hand-duplicated VIES/VAT-ID/PLZ/tracking logic with shared package

Deletes app/lib/vies.ts, vatId.ts, tracking.ts in favor of the newly unified @einfach-produktiv/invoicing modules — fixes the actual PLZ inconsistency (this repo already validated per-country digit counts; the backend hardcoded German-only) rather than just deduplicating code.

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

app/api/account/profile/route.ts

@@ -1,6 +1,6 @@
 import { NextResponse } from "next/server";
 import { getSessionCustomer, updateCustomerProfile } from "../../../lib/customerAuth";
-import { normalizeVatId, isValidVatId } from "../../../lib/vatId";
+import { normalizeVatId, isValidVatId } from "@einfach-produktiv/invoicing";
 
 export async function GET() {
   const session = await getSessionCustomer();

app/api/checkout/route.ts

@@ -8,8 +8,8 @@ import { fetchProductsBySlug } from "../../lib/productsServer";
 import { describeBundleContents } from "../../lib/bundleContents";
 import { sendCriticalAlert } from "../../lib/alertAdmin";
 import { sendOrderConfirmationEmail } from "../../lib/orderEmail";
-import { normalizeVatId, isValidVatId } from "../../lib/vatId";
-import { checkVatIdViaVies } from "../../lib/vies";
+import { normalizeVatId, isValidVatId } from "@einfach-produktiv/invoicing";
+import { checkVatIdViaVies } from "@einfach-produktiv/invoicing/vies";
 import { computeExemptTotals, destinationCountry, isExemptionEligibleCountry } from "../../lib/vatExemption";
 import { upsertNewsletterContact } from "../../lib/brevo";
 import { paymentProvider, isPaymentTestMode } from "../../lib/payments";

app/api/checkout/validate-vat/route.ts

@@ -1,6 +1,6 @@
 import { NextResponse } from "next/server";
-import { normalizeVatId, isValidVatId } from "../../../lib/vatId";
-import { checkVatIdViaVies } from "../../../lib/vies";
+import { normalizeVatId, isValidVatId } from "@einfach-produktiv/invoicing";
+import { checkVatIdViaVies } from "@einfach-produktiv/invoicing/vies";
 
 // Called from CheckoutContent.tsx on the USt-IdNr. field's blur, whenever
 // the billing country is Österreich — the only cross-border-EU option this