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/lib/invoiceData.ts

@@ -1,15 +1,15 @@
-import { getInvoiceSettings, type InvoiceSettings } from "./payload";
+import { getCompanySettings, type CompanySettings } from "./payload";
 import { renderInvoicePdf, type InvoiceOrder } from "./invoicePdf";
 import { renderCorrectionInvoicePdf, type CorrectionInvoiceKind, type CorrectionInvoiceOrder } from "./correctionInvoicePdf";
 
-export type { InvoiceSettings };
+export type { CompanySettings };
 
 // Fetched once by callers that need seller data for more than one purpose
 // in the same request (e.g. orderEmail.ts also needs it for the email
-// footer's companyLine) — avoids a second identical invoice-settings round
-// trip, unlike calling getInvoiceSettings() again inside each generator.
-export async function getSellerForInvoice(): Promise<InvoiceSettings | null> {
-  return getInvoiceSettings();
+// footer's companyLine) — avoids a second identical company-settings round
+// trip, unlike calling getCompanySettings() again inside each generator.
+export async function getSellerForInvoice(): Promise<CompanySettings | null> {
+  return getCompanySettings();
 }
 
 // Shared by app/lib/orderEmail.ts (checkout — attaches to the confirmation
@@ -18,9 +18,9 @@ export async function getSellerForInvoice(): Promise<InvoiceSettings | null> {
 // later always matches what they were emailed. `seller` is passed in
 // rather than fetched here, so a caller that already has it (see above)
 // doesn't fetch it twice.
-export async function generateInvoicePdf(order: InvoiceOrder, seller: InvoiceSettings | null): Promise<Buffer | null> {
+export async function generateInvoicePdf(order: InvoiceOrder, seller: CompanySettings | null): Promise<Buffer | null> {
   if (!seller) {
-    console.error("generateInvoicePdf: no invoice-settings row found for tenant");
+    console.error("generateInvoicePdf: no company-settings row found for tenant");
     return null;
   }
   return renderInvoicePdf(order, seller);
@@ -35,10 +35,10 @@ export async function generateInvoicePdf(order: InvoiceOrder, seller: InvoiceSet
 export async function generateCorrectionInvoicePdf(
   kind: CorrectionInvoiceKind,
   order: CorrectionInvoiceOrder,
-  seller: InvoiceSettings | null,
+  seller: CompanySettings | null,
 ): Promise<Buffer | null> {
   if (!seller) {
-    console.error("generateCorrectionInvoicePdf: no invoice-settings row found for tenant");
+    console.error("generateCorrectionInvoicePdf: no company-settings row found for tenant");
     return null;
   }
   return renderCorrectionInvoicePdf(kind, order, seller);