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/company-settings-preview/components/LiveCompanySettingsPreviewClient.tsx

@@ -31,7 +31,12 @@ export function LiveCompanySettingsPreviewClient({ initialSettings }: { initialS
 
   return (
     <PDFViewer style={{ width: "100%", height: "100vh", border: "none" }}>
-      <InvoiceDocument order={SAMPLE_INVOICE_ORDER} seller={data} />
+      {/* kleinunternehmer isn't part of InvoiceSeller (it's snapshotted
+          per-order, not read live off the seller — see invoicePdf.tsx's
+          own comment) — merged onto the sample order here only, so an
+          admin toggling the checkbox sees the §19 notice reflected live
+          without this preview needing its own separate mechanism. */}
+      <InvoiceDocument order={{ ...SAMPLE_INVOICE_ORDER, kleinunternehmer: data.kleinunternehmer }} seller={data} />
     </PDFViewer>
   );
 }