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/components/CheckoutContent.tsx

@@ -97,6 +97,7 @@ export function CheckoutContent({
   trustBadges,
   shippingSettings,
   defaultTaxRate,
+  kleinunternehmer,
   customerEmail,
   savedProfile,
 }: {
@@ -113,6 +114,11 @@ export function CheckoutContent({
   shippingSettings: ShippingSettings;
   /** Tenant's default VAT rate, same role as CartContent's own prop. */
   defaultTaxRate: number;
+  /** §19 UStG — same role as CartContent's own prop. Also forces the VIES
+   * exemption preview off below: a Kleinunternehmer never charges VAT on
+   * anything, domestic or cross-border, so there's nothing left for the
+   * intra-community exemption to zero-rate. */
+  kleinunternehmer: boolean;
   /** From the checkout page's own session read (app/lib/customerAuth.ts) —
    * null means no account is logged in yet, which flips "1. Rechnungsadresse"
    * into inline-registration mode (password field shown, account created on
@@ -341,7 +347,7 @@ export function CheckoutContent({
   // when set, the billing country otherwise — the exemption depends on
   // where the goods actually move to, not necessarily the invoice address.
   const buyerDestinationCountry = destinationCountry(country, hasDifferentShippingAddress, shippingCountry);
-  const vatExemptPreview = vatIdViesStatus === "valid" && isExemptionEligibleCountry(buyerDestinationCountry);
+  const vatExemptPreview = !kleinunternehmer && vatIdViesStatus === "valid" && isExemptionEligibleCountry(buyerDestinationCountry);
   const exemptTotalsPreview = vatExemptPreview
     ? computeExemptTotals(
         items.map(({ entry, product }) => ({
@@ -538,6 +544,7 @@ export function CheckoutContent({
         discountCode: data.discountCode,
         discountAmount: data.discountAmount,
         vatExempt: Boolean(data.vatExempt),
+        kleinunternehmer: Boolean(data.kleinunternehmer),
       };
       try {
         window.sessionStorage.setItem(ORDER_KEY, JSON.stringify(snapshot));
@@ -1160,7 +1167,7 @@ export function CheckoutContent({
                     {entry.variant ? ` (${entry.variant})` : ""}
                   </p>
                   <p className="text-label text-text-muted">
-                    {entry.qty} × {formatPrice(unitPrice)} <span>inkl. {taxRate}% MwSt.</span>
+                    {entry.qty} × {formatPrice(unitPrice)} {!kleinunternehmer && <span>inkl. {taxRate}% MwSt.</span>}
                   </p>
                 </div>
                 <p className="text-body-sm text-text-primary whitespace-nowrap">{formatPrice(entry.qty * unitPrice)}</p>
@@ -1235,7 +1242,9 @@ export function CheckoutContent({
                 <span className="flex-1" />
                 <span className="font-bold text-h-small text-text-primary">{formatPrice(displayTotal)}</span>
               </div>
-              {vatExemptPreview ? (
+              {kleinunternehmer ? (
+                <p className="text-label text-text-muted">Gemäß § 19 UStG wird keine Umsatzsteuer berechnet.</p>
+              ) : vatExemptPreview ? (
                 <p className="text-label text-text-muted">Steuerfreie innergemeinschaftliche Lieferung (§4 Nr. 1b UStG)</p>
               ) : (
                 <VatBreakdown groups={taxBreakdown} />