Rename invoice-settings to company-settings, add its own Live Preview, and refine invoice PDF layout

Company data now has its own Payload admin group and a live in-browser
PDF preview (react-pdf's PDFViewer) instead of just a plain settings
form. Invoice header is a brand-colored rule instead of a filled band,
and the footer is now pinned to the page bottom instead of following
content flow.

app/company-settings-preview/page.tsx

@@ -0,0 +1,34 @@
+import type { Metadata } from "next";
+import { draftMode } from "next/headers";
+import { getCompanySettings, type CompanySettings } from "../lib/payload";
+import { LiveCompanySettingsPreviewClient } from "./components/LiveCompanySettingsPreviewClient";
+
+export const metadata: Metadata = {
+  title: "Firmendaten-Vorschau",
+  robots: { index: false, follow: false },
+};
+
+const FALLBACK: CompanySettings = {
+  sellerName: "",
+  sellerStreet: "",
+  sellerZip: "",
+  sellerCity: "",
+  sellerCountry: "",
+  sellerEmail: "",
+  vatId: "",
+  taxRatePercent: 19,
+  bankDetails: null,
+};
+
+// Entered exclusively via CompanySettings.ts's admin.livePreview.url (a
+// Payload-admin-only iframe target, see buildPreviewUrl()/api/preview) —
+// not a page a real visitor would ever land on. Unlike email-templates
+// there's no draft/published distinction here (company-settings has no
+// content-versioning concept, it's just the current row) — the initial
+// fetch is the same live data getCompanySettings() always returns,
+// useLivePreview() takes over from there as the admin edits fields.
+export default async function CompanySettingsPreviewPage() {
+  await draftMode();
+  const initialSettings = (await getCompanySettings()) ?? FALLBACK;
+  return <LiveCompanySettingsPreviewClient initialSettings={initialSettings} />;
+}