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/todo-cards/components/Pricing.tsx

@@ -1,7 +1,7 @@
 import Image from "next/image";
 import { AddToCartButton } from "../../components/AddToCartButton";
 import { Reveal } from "../../components/Reveal";
-import { getProductBySlug, getShippingSettings, getDefaultTaxRatePercent } from "../../lib/payload";
+import { getProductBySlug, getShippingSettings, getDefaultTaxRatePercent, getKleinunternehmer } from "../../lib/payload";
 import { formatPrice, discountPercent } from "../../lib/format";
 import { effectiveTaxRate } from "../../lib/cartTotals";
 
@@ -18,10 +18,11 @@ const bullets = [
 // reasoning; the bullet list stays hand-written since it's spec detail,
 // not something the Products collection models.
 export async function Pricing() {
-  const [product, shipping, defaultTaxRate] = await Promise.all([
+  const [product, shipping, defaultTaxRate, kleinunternehmer] = await Promise.all([
     getProductBySlug("todo-karten"),
     getShippingSettings(),
     getDefaultTaxRatePercent(),
+    getKleinunternehmer(),
   ]);
   if (!product) return null;
   const discount = discountPercent(product.price, product.compareAtPrice);
@@ -87,7 +88,7 @@ export async function Pricing() {
               )}
               <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>

app/todo-cards/components/TodoKartenHero.tsx

@@ -2,7 +2,7 @@ import Link from "next/link";
 import Image from "next/image";
 import { AddToCartButton } from "../../components/AddToCartButton";
 import { Reveal } from "../../components/Reveal";
-import { getProductBySlug, getShippingSettings, getDefaultTaxRatePercent } from "../../lib/payload";
+import { getProductBySlug, getShippingSettings, getDefaultTaxRatePercent, getKleinunternehmer } from "../../lib/payload";
 import { formatPrice, discountPercent } from "../../lib/format";
 import { effectiveTaxRate } from "../../lib/cartTotals";
 
@@ -19,10 +19,11 @@ const checklist = [
 // §1 Abs.1 Nr.8 EGBGB's delivery-date disclosure needs to sit next to every
 // buy button, not just one of them.
 export async function TodoKartenHero() {
-  const [product, shipping, defaultTaxRate] = await Promise.all([
+  const [product, shipping, defaultTaxRate, kleinunternehmer] = await Promise.all([
     getProductBySlug("todo-karten"),
     getShippingSettings(),
     getDefaultTaxRatePercent(),
+    getKleinunternehmer(),
   ]);
   const discount = product ? discountPercent(product.price, product.compareAtPrice) : null;
   const taxRate = product ? effectiveTaxRate(product, defaultTaxRate) : null;
@@ -107,7 +108,7 @@ export async function TodoKartenHero() {
                     <p className="text-body text-text-muted line-through">{formatPrice(product.compareAtPrice!)}</p>
                   )}
                   <p className="font-bold text-h3 text-text-primary">{formatPrice(product.price)}</p>
-                  <p className="text-label text-text-muted">inkl. {taxRate}% MwSt.</p>
+                  {!kleinunternehmer && <p className="text-label text-text-muted">inkl. {taxRate}% MwSt.</p>}
                 </div>
               )}
               <p className="text-label text-text-muted">