Add Kleinunternehmerregelung (§19 UStG) support end-to-end

Checkout forces 0% VAT without de-grossing prices when the tenant is a
Kleinunternehmer (a business decision, not just an engineering default —
unlike the existing intra-community VAT exemption, which does de-gross).
Snapshotted onto the order at checkout time so a later toggle of the
company-settings checkbox never rewrites an already-issued invoice's tax
treatment — same reasoning as the existing vatExempt field.

Threaded through: checkout route, order creation/confirmation email,
on-demand invoice/Storno/Gutschrift downloads, the Bestellbestätigung
page, and the account order-detail page. The four storefront "inkl. X%
MwSt." price hints (shop grid, cart upsell, ToDo-Karten landing page,
homepage spotlight) drop that clause live when the setting is on. The
company-settings Live Preview reflects the checkbox in real time too.

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, getShippingCountries, getPaymentMethods, getCartTrustBadges, getShippingSettings, getDefaultTaxRatePercent } from "../lib/payload";
+import { getShippingMethods, getShippingCountries, getPaymentMethods, getCartTrustBadges, getShippingSettings, getDefaultTaxRatePercent, getKleinunternehmer } from "../lib/payload";
 import { getSessionCustomer, getCustomerProfile } from "../lib/customerAuth";
 
 // robots: noindex — transactional page, same reasoning as /cart.
@@ -16,13 +16,14 @@ export const metadata: Metadata = {
 };
 
 export default async function CheckoutPage() {
-  const [shippingMethods, shippingCountries, paymentMethods, trustBadges, shippingSettings, defaultTaxRate, session] = await Promise.all([
+  const [shippingMethods, shippingCountries, paymentMethods, trustBadges, shippingSettings, defaultTaxRate, kleinunternehmer, session] = await Promise.all([
     getShippingMethods(),
     getShippingCountries(),
     getPaymentMethods(),
     getCartTrustBadges(),
     getShippingSettings(),
     getDefaultTaxRatePercent(),
+    getKleinunternehmer(),
     getSessionCustomer(),
   ]);
   // Full profile (incl. saved address) only fetched when a session exists
@@ -39,6 +40,7 @@ export default async function CheckoutPage() {
           trustBadges={trustBadges}
           shippingSettings={shippingSettings}
           defaultTaxRate={defaultTaxRate}
+          kleinunternehmer={kleinunternehmer}
           customerEmail={session?.customer.email ?? null}
           savedProfile={profile}
         />