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/components/LiveCompanySettingsPreviewClient.tsx

@@ -0,0 +1,37 @@
+"use client";
+
+import dynamic from "next/dynamic";
+import { useLivePreview } from "@payloadcms/live-preview-react";
+import { InvoiceDocument, SAMPLE_INVOICE_ORDER } from "../../lib/invoicePdf";
+import type { CompanySettings } from "../../lib/payload";
+
+const PAYLOAD_URL = process.env.NEXT_PUBLIC_PAYLOAD_URL || "https://payload.mk360.de";
+
+// @react-pdf/renderer's PDFViewer renders into an <iframe> via direct DOM
+// access — it has to be excluded from the server render pass entirely
+// (ssr: false), unlike the HTML-string email previews elsewhere in this
+// app, which can render server-side fine since they're just
+// dangerouslySetInnerHTML.
+const PDFViewer = dynamic(() => import("@react-pdf/renderer").then((mod) => mod.PDFViewer), { ssr: false });
+
+// Same useLivePreview() mechanism as LiveEmailPreviewClient.tsx — connects
+// to the Payload admin's iframe via postMessage and updates `data` as the
+// admin edits company-settings fields, no save required. Renders through
+// the exact same InvoiceDocument component the real invoice PDF uses
+// (app/lib/invoicePdf.tsx), against a fixed sample order
+// (SAMPLE_INVOICE_ORDER) — there's no "current" real order to preview
+// against generically, same reasoning as the email-templates preview's
+// own SAMPLE_ORDER.
+export function LiveCompanySettingsPreviewClient({ initialSettings }: { initialSettings: CompanySettings }) {
+  const { data } = useLivePreview<CompanySettings>({
+    initialData: initialSettings,
+    serverURL: PAYLOAD_URL,
+    depth: 0,
+  });
+
+  return (
+    <PDFViewer style={{ width: "100%", height: "100vh", border: "none" }}>
+      <InvoiceDocument order={SAMPLE_INVOICE_ORDER} seller={data} />
+    </PDFViewer>
+  );
+}