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/components/ProductSpotlight.tsx

@@ -2,7 +2,7 @@ import Image from "next/image";
 import Link from "next/link";
 import { AddToCartButton } from "./AddToCartButton";
 import { Reveal } from "./Reveal";
-import { getSpotlightProduct, getShippingSettings, getDefaultTaxRatePercent } from "../lib/payload";
+import { getSpotlightProduct, getShippingSettings, getDefaultTaxRatePercent, getKleinunternehmer } from "../lib/payload";
 import { formatPrice, discountPercent } from "../lib/format";
 import { effectiveTaxRate } from "../lib/cartTotals";
 
@@ -23,10 +23,11 @@ import { effectiveTaxRate } from "../lib/cartTotals";
  * see Products.ts), not duplicated here as hardcoded literals.
  */
 export async function ProductSpotlight() {
-  const [product, shipping, defaultTaxRate] = await Promise.all([
+  const [product, shipping, defaultTaxRate, kleinunternehmer] = await Promise.all([
     getSpotlightProduct(),
     getShippingSettings(),
     getDefaultTaxRatePercent(),
+    getKleinunternehmer(),
   ]);
   if (!product) return null;
 
@@ -85,7 +86,7 @@ export async function ProductSpotlight() {
               )}
               <p className="font-bold text-h3 text-text-primary">{formatPrice(product.price)}</p>
             </div>
-            <p className="text-label text-text-muted">inkl. {taxRate}% MwSt. zzgl. Versand</p>
+            <p className="text-label text-text-muted">{kleinunternehmer ? "zzgl. Versand" : `inkl. ${taxRate}% MwSt. zzgl. Versand`}</p>
             <p className="text-label text-text-muted">
               Lieferzeit: {shipping.totalDays.min}–{shipping.totalDays.max} Werktage innerhalb Deutschlands
             </p>