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/api/checkout/route.ts

@@ -1,6 +1,6 @@
 import { NextResponse } from "next/server";
 import type { CartItem } from "../../lib/cart";
-import { getShippingMethods, getPaymentMethods, getInvoiceSettings } from "../../lib/payload";
+import { getShippingMethods, getPaymentMethods, getCompanySettings } from "../../lib/payload";
 import { validateDiscountCode, redeemDiscountCode } from "../../lib/discountServer";
 import { createOrder } from "../../lib/orderServer";
 import { getSessionCustomer, registerCustomer, setSessionCookie, type CustomerSummary } from "../../lib/customerAuth";
@@ -102,8 +102,8 @@ export async function POST(request: Request) {
   }
 
   // Re-price everything server-side — never trust client-submitted prices.
-  const [productsBySlug, invoiceSettings] = await Promise.all([fetchProductsBySlug(), getInvoiceSettings()]);
-  const defaultTaxRate = invoiceSettings?.taxRatePercent ?? 19;
+  const [productsBySlug, companySettings] = await Promise.all([fetchProductsBySlug(), getCompanySettings()]);
+  const defaultTaxRate = companySettings?.taxRatePercent ?? 19;
   const items: {
     productId: number;
     productName: string;