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/checkoutDraft.ts

@@ -14,6 +14,11 @@ export type CheckoutDraft = {
   firstName: string;
   lastName: string;
   email: string;
+  // Optional B2B fields — see CheckoutContent.tsx's own comment on why
+  // they sit here (right next to the Rechnungsadresse fields, not a
+  // separate persisted concept).
+  companyName: string;
+  vatId: string;
   // Rechnungsadresse is always a plain street address now — no
   // deliveryMethod/packstationNumber/postNumber here, only on the
   // shipping* override fields below (see CheckoutContent.tsx).