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/bestellbestaetigung/components/BestellbestaetigungContent.tsx

@@ -33,7 +33,8 @@ function parseOrderSnapshot(raw: string): OrderSnapshot | null {
       typeof data.paymentMethodTitle !== "string" ||
       (data.discountCode !== null && typeof data.discountCode !== "string") ||
       typeof data.discountAmount !== "number" ||
-      typeof data.vatExempt !== "boolean"
+      typeof data.vatExempt !== "boolean" ||
+      typeof data.kleinunternehmer !== "boolean"
     ) {
       return null;
     }
@@ -229,7 +230,7 @@ export function BestellbestaetigungContent({ defaultTaxRate }: { defaultTaxRate:
                       {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)} {!order.kleinunternehmer && <span>inkl. {taxRate}% MwSt.</span>}
                     </p>
                   </div>
                   <p className="text-body-sm text-text-primary whitespace-nowrap">
@@ -281,7 +282,9 @@ export function BestellbestaetigungContent({ defaultTaxRate }: { defaultTaxRate:
                   <span className="flex-1" />
                   <span className="font-bold text-h-small text-text-primary">{formatPrice(total)}</span>
                 </div>
-                {order.vatExempt ? (
+                {order.kleinunternehmer ? (
+                  <p className="text-label text-text-muted">Gemäß § 19 UStG wird keine Umsatzsteuer berechnet.</p>
+                ) : order.vatExempt ? (
                   <p className="text-label text-text-muted">Steuerfreie innergemeinschaftliche Lieferung (§4 Nr. 1b UStG)</p>
                 ) : (
                   <VatBreakdown groups={taxBreakdown} />