Document the shared-modules section pointing to @einfach-produktiv/invoicing

Notes what moved into the shared package (VIES, VAT-ID, PLZ, carrier-tracking) and why — this repo's README previously didn't call out the modularization/dedup pass at all, only individual file references scattered through other sections.

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

README.md

@@ -21,6 +21,18 @@ specific to this project.
   via an httpOnly session cookie this app mints itself; see "Orders &
   customer accounts" below.
 
+## Shared modules
+
+`@einfach-produktiv/invoicing` (a separate repo, consumed as a git
+dependency — see "Invoice PDFs" below) is where framework-agnostic logic
+shared with the Payload backend lives, so it's written once instead of
+maintained as two independently-drifting copies. Beyond invoice/e-invoice
+PDF generation and tax-breakdown math, it also holds VIES VAT-ID checking,
+VAT-ID format validation, PLZ digit-count validation, and shipping-carrier
+tracking-label logic — all previously hand-duplicated `app/lib/*.ts` files
+in this repo, now imported from the shared package instead. See that
+package's own README for what's in it and why each piece was unified.
+
 ## Getting started
 
 ```bash