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/customerAuth.ts

@@ -199,6 +199,10 @@ export type CustomerAddress = {
   zip: string | null;
   city: string | null;
   country: string | null;
+  // Optional B2B profile default — see Customers.ts's own comment. Prefills
+  // /checkout's Firma/USt-IdNr. fields for a returning customer.
+  companyName: string | null;
+  vatId: string | null;
 };
 
 export type CustomerProfile = CustomerSummary & CustomerAddress;
@@ -217,6 +221,8 @@ type PayloadCustomerMe = {
   zip: string | null;
   city: string | null;
   country: string | null;
+  companyName: string | null;
+  vatId: string | null;
   cart: { product: number; productSlug: string; quantity: number; variantName: string | null }[] | null;
 };
 
@@ -243,6 +249,8 @@ export async function getCustomerProfile(token: string): Promise<CustomerProfile
     zip: u.zip,
     city: u.city,
     country: u.country,
+    companyName: u.companyName,
+    vatId: u.vatId,
   };
 }
 
@@ -259,6 +267,8 @@ export async function updateCustomerProfile(
     zip: string;
     city: string;
     country: string;
+    companyName?: string;
+    vatId?: string;
   },
 ): Promise<{ ok: true } | { ok: false; reason: string }> {
   const res = await fetch(`${PAYLOAD_URL}/api/customers/${customerId}`, {