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/konto/bestellungen/[orderNumber]/page.tsx

@@ -100,7 +100,9 @@ export default async function KontoBestellungDetailPage({ params }: { params: Pr
             {order.vatId && (
               <p className="text-body-sm text-text-muted">
                 USt-IdNr. {order.vatId}
-                {order.vatExempt && " · steuerfreie innergemeinschaftliche Lieferung"}
+                {order.kleinunternehmer
+                  ? " · Kleinunternehmer gem. § 19 UStG"
+                  : order.vatExempt && " · steuerfreie innergemeinschaftliche Lieferung"}
               </p>
             )}
           </div>
@@ -131,7 +133,7 @@ export default async function KontoBestellungDetailPage({ params }: { params: Pr
                     {item.quantity} × {item.productName}
                     {item.variantName ? ` (${item.variantName})` : ""}
                   </p>
-                  <p className="text-label text-text-muted">inkl. {item.taxRatePercent}% MwSt.</p>
+                  {!order.kleinunternehmer && <p className="text-label text-text-muted">inkl. {item.taxRatePercent}% MwSt.</p>}
                   {item.bundleContents && <p className="text-label text-text-muted">{item.bundleContents}</p>}
                   {item.returnQuantity > 0 && (
                     <p className="text-label text-text-muted">davon {item.returnQuantity} zurückgesendet</p>
@@ -174,7 +176,11 @@ export default async function KontoBestellungDetailPage({ params }: { params: Pr
                 <span className="flex-1" />
                 <span className="font-bold text-h-small text-text-primary">{formatPrice(order.total)}</span>
               </div>
-              <VatBreakdown groups={taxBreakdown} />
+              {order.kleinunternehmer ? (
+                <p className="text-label text-text-muted">Gemäß § 19 UStG wird keine Umsatzsteuer berechnet.</p>
+              ) : (
+                <VatBreakdown groups={taxBreakdown} />
+              )}
             </div>
           </div>