Feed checkout's country selects from Payload instead of a hardcoded list

Both the billing and shipping-override country selects, plus PLZ
maxLength/pattern validation, now read from the new shipping-countries
collection (getShippingCountries()) rather than a hardcoded
Deutschland/Österreich(/Schweiz) array. Lets an admin add or reorder
destination countries without a frontend deploy.

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

app/checkout/page.tsx

@@ -2,7 +2,7 @@ import type { Metadata } from "next";
 import { CheckoutContent } from "./components/CheckoutContent";
 import { TrustRow } from "../components/TrustRow";
 import { Footer } from "../components/Footer";
-import { getShippingMethods, getPaymentMethods, getCartTrustBadges, getShippingSettings, getDefaultTaxRatePercent } from "../lib/payload";
+import { getShippingMethods, getShippingCountries, getPaymentMethods, getCartTrustBadges, getShippingSettings, getDefaultTaxRatePercent } from "../lib/payload";
 import { getSessionCustomer, getCustomerProfile } from "../lib/customerAuth";
 
 // robots: noindex — transactional page, same reasoning as /cart.
@@ -16,8 +16,9 @@ export const metadata: Metadata = {
 };
 
 export default async function CheckoutPage() {
-  const [shippingMethods, paymentMethods, trustBadges, shippingSettings, defaultTaxRate, session] = await Promise.all([
+  const [shippingMethods, shippingCountries, paymentMethods, trustBadges, shippingSettings, defaultTaxRate, session] = await Promise.all([
     getShippingMethods(),
+    getShippingCountries(),
     getPaymentMethods(),
     getCartTrustBadges(),
     getShippingSettings(),
@@ -33,6 +34,7 @@ export default async function CheckoutPage() {
       <main className="flex flex-col flex-1 bg-bg-base">
         <CheckoutContent
           shippingMethods={shippingMethods}
+          shippingCountries={shippingCountries}
           paymentMethods={paymentMethods}
           trustBadges={trustBadges}
           shippingSettings={shippingSettings}