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/api/account/orders/[orderNumber]/correction-invoice/route.ts

@@ -37,6 +37,7 @@ export async function GET(request: Request, { params }: { params: Promise<{ orde
       companyName: order.companyName,
       vatId: order.vatId,
       vatExempt: order.vatExempt,
+      kleinunternehmer: order.kleinunternehmer,
       deliveryMethod: order.deliveryMethod,
       street: order.street,
       packstationNumber: order.packstationNumber,

app/api/account/orders/[orderNumber]/invoice/route.ts

@@ -34,6 +34,7 @@ export async function GET(request: Request, { params }: { params: Promise<{ orde
       companyName: order.companyName,
       vatId: order.vatId,
       vatExempt: order.vatExempt,
+      kleinunternehmer: order.kleinunternehmer,
       deliveryMethod: order.deliveryMethod,
       street: order.street,
       packstationNumber: order.packstationNumber,