Add optional Firma/USt-IdNr. fields to checkout and profile

B2B checkout fields, split out from the e-invoicing migration and
picked back up now that it's shipped. Both fields are independently
optional, format-validated (shared regex in lib/vatId.ts, mirrored
server-side in api/checkout and api/account/profile), persisted in
the checkout draft, and saved as a customer profile default that
pre-fills future checkouts. Order/customer snapshot fields land in a
companion Payload backend commit.

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

app/lib/vatId.ts

@@ -0,0 +1,15 @@
+// Mirrors the backend's own USt-IdNr. validation exactly (Orders.ts/
+// Customers.ts/CompanySettings.ts in the Payload repo) — kept as a plain
+// client+server-safe helper here since this repo's frontend needs the same
+// check twice (checkout's instant client-side pattern + api/checkout's own
+// server-side re-validation, same "never trust the client" reasoning as
+// every other checkout field).
+const VAT_ID_PATTERN = /^[A-Z]{2}[A-Z0-9]{2,12}$/;
+
+export function normalizeVatId(value: string): string {
+  return value.toUpperCase().trim();
+}
+
+export function isValidVatId(value: string): boolean {
+  return VAT_ID_PATTERN.test(value);
+}