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/components/CartContent.tsx

@@ -22,6 +22,7 @@ export function CartContent({
   freeShippingThreshold,
   shippingSettings,
   defaultTaxRate,
+  kleinunternehmer,
   showDiscountField,
 }: {
   trustBadges: TrustBadge[];
@@ -42,6 +43,11 @@ export function CartContent({
    * override taxRatePercent themselves — see lib/cartTotals.ts's
    * effectiveTaxRate(). */
   defaultTaxRate: number;
+  /** §19 UStG — this tenant's company-settings.kleinunternehmer (Payload's
+   * lib/payload.ts's getKleinunternehmer(), same ISR freshness as
+   * defaultTaxRate above). Drops the "inkl. X% MwSt." hints and the VAT
+   * breakdown in favor of the §19 notice below. */
+  kleinunternehmer: boolean;
   /** Whether Payload currently has at least one active discount code at
    * all (lib/discountServer.ts's hasActiveDiscountCode()) — no point
    * showing an open "enter a code" field when nothing could ever validate
@@ -236,7 +242,7 @@ export function CartContent({
                           <span className="text-label text-text-muted line-through">{formatPrice(product.compareAtPrice!)}</span>
                         )}
                         <span className="font-bold text-body-sm text-text-primary">{formatPrice(unitPrice)}</span>
-                        <span className="text-label text-text-muted">inkl. {taxRate}% MwSt.</span>
+                        {!kleinunternehmer && <span className="text-label text-text-muted">inkl. {taxRate}% MwSt.</span>}
                       </p>
                     </div>
                   </div>
@@ -413,7 +419,11 @@ export function CartContent({
                   <span className="flex-1" />
                   <span className="font-bold text-h-small text-text-primary">{formatPrice(total)}</span>
                 </div>
-                <VatBreakdown groups={taxBreakdown} />
+                {kleinunternehmer ? (
+                  <p className="text-label text-text-muted">Gemäß § 19 UStG wird keine Umsatzsteuer berechnet.</p>
+                ) : (
+                  <VatBreakdown groups={taxBreakdown} />
+                )}
               </div>
 
               <Link

app/cart/components/RelatedProducts.tsx

@@ -30,7 +30,7 @@ function pickAvailable(allIds: string[], excludeIds: string[], keep: string[], c
   return [...keep, ...pickRandom(allIds, [...excludeIds, ...keep], missing)];
 }
 
-export function RelatedProducts({ defaultTaxRate }: { defaultTaxRate: number }) {
+export function RelatedProducts({ defaultTaxRate, kleinunternehmer }: { defaultTaxRate: number; kleinunternehmer: boolean }) {
   const cart = useCart();
   const products = useProducts();
   // Cart/checkout resolve any product regardless of `active` (see
@@ -188,7 +188,7 @@ export function RelatedProducts({ defaultTaxRate }: { defaultTaxRate: number })
                   <span className="text-label text-text-muted line-through">{formatPrice(product.compareAtPrice!)}</span>
                 )}
                 <span className="font-bold text-h4 text-text-primary">{formatPrice(product.price)}</span>
-                <span className="text-label text-text-muted">inkl. {taxRate}% MwSt.</span>
+                {!kleinunternehmer && <span className="text-label text-text-muted">inkl. {taxRate}% MwSt.</span>}
               </p>
               {/* Always rendered, text conditional — min-h reserves this
                   line's height in both states so cards in the same row