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/lib/payload.ts

@@ -471,6 +471,46 @@ export async function getShippingMethods(): Promise<ShippingMethod[]> {
   }));
 }
 
+// Feeds /checkout's "Land" <select> (both the billing address and the
+// optional shipping-address override) and its PLZ digit-count validation
+// — previously a hardcoded array + PLZ_DIGITS map in CheckoutContent.tsx
+// itself. Which countries are actually deliverable can now change without
+// a code deploy (e.g. temporarily dropping Schweiz — no customs/export-
+// invoice handling exists for it yet). Deliberately unrelated to VAT-
+// exemption eligibility (lib/vatExemption.ts's isExemptionEligibleCountry(),
+// still hardcoded to "Österreich") — that's a legal/tax-law question, not
+// a shipping-logistics one, and stays in code on purpose.
+export type ShippingCountry = {
+  name: string;
+  plzDigits: number;
+};
+
+type PayloadShippingCountry = ShippingCountry & { active: boolean };
+
+export async function getShippingCountries(): Promise<ShippingCountry[]> {
+  const params = new URLSearchParams({
+    "where[tenant.slug][equals]": TENANT_SLUG,
+    "where[active][equals]": "true",
+    sort: "sortOrder",
+    limit: "20",
+  });
+
+  const res = await fetch(`${PAYLOAD_URL}/api/shipping-countries?${params}`, {
+    next: { revalidate: 60 },
+  });
+  if (!res.ok) {
+    console.error(`getShippingCountries: Payload returned ${res.status} ${res.statusText}`);
+    return [];
+  }
+
+  const data: { docs?: PayloadShippingCountry[] } = await res.json();
+  const docs = Array.isArray(data.docs) ? data.docs : [];
+  return docs.map((doc) => ({
+    name: doc.name,
+    plzDigits: doc.plzDigits,
+  }));
+}
+
 export type ShippingSettings = {
   handlingDays: { min: number; max: number };
   transitDays: { min: number; max: number };