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/konto/profil/components/ProfileForm.tsx

@@ -45,6 +45,8 @@ export function ProfileForm({ profile }: { profile: CustomerProfile }) {
       zip: String(form.get("zip") ?? ""),
       city: String(form.get("city") ?? ""),
       country: String(form.get("country") ?? ""),
+      companyName: String(form.get("companyName") ?? "") || undefined,
+      vatId: String(form.get("vatId") ?? "") || undefined,
     };
 
     try {
@@ -83,6 +85,22 @@ export function ProfileForm({ profile }: { profile: CustomerProfile }) {
           <Field label="Nachname" name="lastName" type="text" defaultValue={profile.lastName} required />
         </div>
 
+        {/* Optional B2B fields — prefills /checkout's own Firma/USt-IdNr.
+            fields, same "profile default, order keeps its own snapshot"
+            split as the address fields below (see Customers.ts). */}
+        <div className="flex flex-col sm:flex-row gap-4 w-full">
+          <Field label="Firma (optional)" name="companyName" type="text" defaultValue={profile.companyName ?? ""} />
+          <Field
+            label="USt-IdNr. (optional)"
+            name="vatId"
+            type="text"
+            defaultValue={profile.vatId ?? ""}
+            placeholder="DE123456789"
+            pattern="[A-Za-z]{2}[A-Za-z0-9]{2,12}"
+            title="EU-Format: 2 Buchstaben Länderpräfix + bis zu 12 alphanumerische Zeichen, z. B. DE123456789."
+          />
+        </div>
+
         <div className="w-full flex flex-col gap-2 items-start">
           <span className="text-label text-text-muted">Lieferart</span>
           <div className="flex w-full max-w-sm rounded-sm border border-border overflow-hidden">