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/api/checkout/route.ts

@@ -8,6 +8,7 @@ import { fetchProductsBySlug } from "../../lib/productsServer";
 import { describeBundleContents } from "../../lib/bundleContents";
 import { sendCriticalAlert } from "../../lib/alertAdmin";
 import { sendOrderConfirmationEmail } from "../../lib/orderEmail";
+import { normalizeVatId, isValidVatId } from "../../lib/vatId";
 
 // Plain float arithmetic on money (quantity × unitPrice summed across
 // lines, a percent discount, subtracting/adding those together) drifts
@@ -30,6 +31,8 @@ type CheckoutBody = {
   lastName: string;
   email: string;
   password?: string;
+  companyName?: string;
+  vatId?: string;
   deliveryMethod: "address" | "packstation";
   street?: string;
   packstationNumber?: string;
@@ -85,6 +88,15 @@ export async function POST(request: Request) {
   if (body.deliveryMethod === "packstation" && (!body.packstationNumber || !body.postNumber)) {
     return NextResponse.json({ ok: false, reason: "Bitte Packstation- und Postnummer angeben." }, { status: 400 });
   }
+  // Optional — only format-checked when actually provided, same "never
+  // trust the client" reasoning as every other checkout field re-validated
+  // here. Normalized the same way Orders.ts's own field does (uppercase +
+  // trim), so the snapshot on the order matches what would've been
+  // accepted directly through the Payload admin.
+  const normalizedVatId = body.vatId ? normalizeVatId(body.vatId) : undefined;
+  if (normalizedVatId && !isValidVatId(normalizedVatId)) {
+    return NextResponse.json({ ok: false, reason: "Ungültiges USt-IdNr.-Format (z. B. DE123456789)." }, { status: 400 });
+  }
   if (body.hasDifferentShippingAddress) {
     if (!body.shippingFirstName || !body.shippingLastName || !body.shippingZip || !body.shippingCity || !body.shippingCountry) {
       return NextResponse.json({ ok: false, reason: "Bitte alle Felder der Lieferadresse ausfüllen." }, { status: 400 });
@@ -201,6 +213,8 @@ export async function POST(request: Request) {
     customerFirstName: body.firstName,
     customerLastName: body.lastName,
     customerEmail: body.email,
+    companyName: body.companyName || undefined,
+    vatId: normalizedVatId,
     deliveryMethod: body.deliveryMethod,
     street: body.street,
     packstationNumber: body.packstationNumber,