Add legalForm-driven Pflichtangaben (register court/number, managing director)

Mirrors the Payload backend's new company-settings.legalForm field: when
present, registerCourt/registerNumber/managingDirector now appear in the
email footer (buildLegalFooterLines) and both invoice PDF footers,
matching §37a HGB / §35a GmbHG requirements for registered legal forms.
A sole proprietorship (the default) renders identically to before —
these fields are only appended when actually set.

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

app/company-settings-preview/page.tsx

@@ -11,6 +11,10 @@ export const metadata: Metadata = {
 
 const FALLBACK: CompanySettings = {
   sellerName: "",
+  legalForm: "sole-proprietorship",
+  registerCourt: null,
+  registerNumber: null,
+  managingDirector: null,
   sellerStreet: "",
   sellerZip: "",
   sellerCity: "",