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/checkout/components/CheckoutContent.tsx

@@ -20,7 +20,7 @@ import { ORDER_KEY, PENDING_ORDER_KEY, type OrderSnapshot } from "../../lib/orde
 import { PaymentStep } from "./PaymentStep";
 import { dispatchAuthChanged } from "../../lib/auth";
 import { readCheckoutDraft, writeCheckoutDraft, clearCheckoutDraft } from "../../lib/checkoutDraft";
-import { normalizeVatId, isValidVatId } from "../../lib/vatId";
+import { normalizeVatId, isValidVatId, isValidPlz, plzInputPattern } from "@einfach-produktiv/invoicing";
 import { computeExemptTotals, destinationCountry, isExemptionEligibleCountry } from "../../lib/vatExemption";
 import { validateEmailFormat } from "../../lib/email";
 import type { ShippingMethod, ShippingCountry, PaymentMethod, TrustBadge, ShippingSettings } from "../../lib/payload";
@@ -37,8 +37,7 @@ import type { CustomerProfile } from "../../lib/customerAuth";
 // here. `?? 4` only matters if a country somehow isn't in the map at all
 // (shouldn't happen — the <select> options are built from the same list).
 function plzPattern(country: string, plzDigitsMap: Record<string, number>): string {
-  const digits = plzDigitsMap[country] ?? 4;
-  return `\\d{${digits}}`;
+  return plzInputPattern(plzDigitsMap[country] ?? 4);
 }
 
 // Same rules as the pattern/required attributes each field already
@@ -52,7 +51,7 @@ function validateRequired(label: string, value: string): string {
 function validateZip(value: string, country: string, plzDigitsMap: Record<string, number>): string {
   if (!value.trim()) return "PLZ ist erforderlich.";
   const digits = plzDigitsMap[country] ?? 4;
-  return new RegExp(`^\\d{${digits}}$`).test(value) ? "" : `PLZ muss aus ${digits} Ziffern bestehen.`;
+  return isValidPlz(value, digits) ? "" : `PLZ muss aus ${digits} Ziffern bestehen.`;
 }
 
 function validatePackstationNumber(value: string): string {