Add innergemeinschaftliche-Lieferung VAT exemption for cross-border B2B

A validated EU business buyer (Österreich, the one cross-border option
this checkout offers) gets the sale zero-rated per §4 Nr. 1b UStG —
but only after a live VIES lookup confirms the VAT ID is actually
registered right now, never from format-validity alone (real
compliance risk otherwise). VIES unreachable fails closed: normal VAT
applies, no guessed exemption.

- lib/vies.ts: calls the EU's public VIES REST API.
- lib/vatExemption.ts: de-grosses item/shipping prices and computes
  the exempt totals; also picks the actual destination country
  (shipping override when set, billing otherwise).
- api/checkout/validate-vat: on-blur live check for instant feedback;
  api/checkout/route.ts re-runs the same check server-side at submit
  as the actual source of truth, and re-prices every line net-of-VAT
  when exempt.
- CheckoutContent.tsx: VIES status + a live exempt-totals preview;
  BestellbestaetigungContent.tsx mirrors it from the persisted
  snapshot. Both blur-validate every other checkout field now too
  (immediate inline errors, not just on submit).
- vatExempt/vatIdValidatedAt threaded through orderServer.ts,
  customerAuth.ts, orderEmail.ts, and both invoice-download routes so
  the invoice PDF and its e-invoice XML (companion payload-repo
  commit) reflect the exemption correctly wherever it's rendered.

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

package-lock.json

@@ -359,7 +359,7 @@
     },
     "node_modules/@einfach-produktiv/invoicing": {
       "version": "0.1.0",
-      "resolved": "git+https://git.mk360.de/Marco/einfach-produktiv-invoicing.git#ceedb4be05465a34d5a8051c8f30d578be1fd7af",
+      "resolved": "git+https://git.mk360.de/Marco/einfach-produktiv-invoicing.git#942b8a86322b165a09e7b91af686b6c3538989f4",
       "dependencies": {
         "@e-invoice-eu/core": "^3.1.1"
       },