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/cart/page.tsx

@@ -4,7 +4,7 @@ import { CartContent } from "./components/CartContent";
 import { RelatedProducts } from "./components/RelatedProducts";
 import { TrustRow } from "../components/TrustRow";
 import { Footer } from "../components/Footer";
-import { getCartTrustBadges, getShippingMethods, getShippingSettings, getDefaultTaxRatePercent } from "../lib/payload";
+import { getCartTrustBadges, getShippingMethods, getShippingSettings, getDefaultTaxRatePercent, getKleinunternehmer } from "../lib/payload";
 import { hasActiveDiscountCode } from "../lib/discountServer";
 
 // robots: noindex — transactional page (mirrors a specific shopper's cart
@@ -20,11 +20,12 @@ export const metadata: Metadata = {
 };
 
 export default async function CartPage() {
-  const [trustBadges, shippingMethods, shipping, defaultTaxRate, showDiscountField] = await Promise.all([
+  const [trustBadges, shippingMethods, shipping, defaultTaxRate, kleinunternehmer, showDiscountField] = await Promise.all([
     getCartTrustBadges(),
     getShippingMethods(),
     getShippingSettings(),
     getDefaultTaxRatePercent(),
+    getKleinunternehmer(),
     hasActiveDiscountCode(),
   ]);
 
@@ -55,10 +56,11 @@ export default async function CartPage() {
             freeShippingThreshold={freeShippingThreshold}
             shippingSettings={shipping}
             defaultTaxRate={defaultTaxRate}
+            kleinunternehmer={kleinunternehmer}
             showDiscountField={showDiscountField}
           />
         </Suspense>
-        <RelatedProducts defaultTaxRate={defaultTaxRate} />
+        <RelatedProducts defaultTaxRate={defaultTaxRate} kleinunternehmer={kleinunternehmer} />
         <TrustRow />
       </main>
       <Footer />