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>
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>
Marco