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>

README.md

@@ -327,6 +327,11 @@ thumbnails per order row (`getProductImagesByIds()` in `app/lib/payload.ts`,
 a plain product-id → image-url lookup separate from the slug-keyed
 catalog, since an order only ever snapshots a numeric product id).
 
+**None of this renders at all for a Kleinunternehmer tenant** (built
+2026-07-24) — see "Kleinunternehmerregelung" below for the full list of
+touched spots and why some read a live setting and others a persisted
+per-order snapshot.
+
 ### Checkout state persistence
 
 `app/lib/checkoutDraft.ts` — `localStorage` under `ep_checkout_draft`,
@@ -833,6 +838,63 @@ turns out unregistered, the seller retroactively owes the VAT itself).
   live from the current catalog, which would show the wrong, VAT-inclusive
   figures for an exempt order).
 
+### Kleinunternehmerregelung (§19 UStG)
+
+Built 2026-07-24. `company-settings.kleinunternehmer` — a standing
+per-tenant setting (Payload backend), not a per-order decision like the
+VAT exemption above — when on, this tenant never charges VAT on anything,
+domestic or cross-border. See the Payload README's own
+"Kleinunternehmerregelung" section for the field/collection side; this one
+covers what changed here.
+
+- **`api/checkout/route.ts`** forces every item's `taxRatePercent` to `0`
+  when `getCompanySettings().kleinunternehmer` is on — deliberately
+  **without** de-grossing `unitPrice` the way `vatExempt` above does
+  (that exemption zero-rates what would otherwise be a positive-rate
+  charge, so de-grossing means the buyer pays less; a Kleinunternehmer
+  never charged VAT on the sale to begin with, so the catalog gross price
+  already *is* the actual net charge — confirmed with the user as the
+  intended business decision, not an engineering default). The VIES
+  lookup/`isExemptionEligibleCountry()` check is skipped entirely in this
+  branch too — there's no VAT for the intra-community rule to exempt
+  either. Snapshotted onto the new order as `Orders.kleinunternehmer`
+  (mirrors `vatExempt`'s own snapshot reasoning — see the Payload README).
+- **Storefront "inkl. X% MwSt." hints** — four spots read the *live*
+  setting (`getKleinunternehmer()` in `app/lib/payload.ts`, same ISR-cached
+  60s freshness as `getDefaultTaxRatePercent()`) and drop the MwSt. clause
+  entirely when it's on, since there's no order yet at that point to
+  snapshot from: `ProductGrid.tsx` (shop grid), `RelatedProducts.tsx`
+  (cart's upsell row), `Pricing.tsx`/`TodoKartenHero.tsx` (ToDo-Karten
+  landing page), `ProductSpotlight.tsx` (homepage). `Pricing.tsx`/
+  `ProductSpotlight.tsx` keep "zzgl. Versand" on its own when the MwSt.
+  clause drops; the other two had no such trailing clause to preserve.
+- **Every already-placed-order display reads the persisted snapshot
+  instead** — `OrderSnapshot.kleinunternehmer` (`app/lib/order.ts`,
+  written into `sessionStorage` at checkout, read by
+  `BestellbestaetigungContent.tsx`) and `CustomerOrderDetail.
+  kleinunternehmer` (`app/lib/customerAuth.ts`, read by
+  `/konto/bestellungen/[orderNumber]`) — never the live company-settings
+  value, for the identical "don't retroactively rewrite an already-issued
+  invoice's tax treatment" reason `vatExempt` already established. Both
+  pages replace the per-item "inkl. X% MwSt." hint and the `VatBreakdown`
+  summary with "Gemäß § 19 UStG wird keine Umsatzsteuer berechnet." —
+  taking precedence over the `vatExempt` note wherever both would
+  otherwise apply. `CheckoutContent.tsx`'s live VIES-exemption *preview*
+  is also gated off (`!kleinunternehmer && ...`) so a Kleinunternehmer
+  tenant never shows a misleading "wird steuerfrei berechnet" preview for
+  VAT that was never going to be charged either way.
+- **On-demand invoice/Stornorechnung/Gutschrift downloads**
+  (`api/account/orders/[orderNumber]/invoice/route.ts` and its
+  `correction-invoice` sibling) thread `order.kleinunternehmer` through to
+  `@einfach-produktiv/invoicing`'s renderers the same way they already
+  thread `vatExempt`.
+- **`app/company-settings-preview`'s Live Preview** merges the live-edited
+  `kleinunternehmer` checkbox onto the fixed `SAMPLE_INVOICE_ORDER` before
+  rendering (`kleinunternehmer` lives on `InvoiceOrder`, not
+  `InvoiceSeller` — see the invoicing package's own README on why), so an
+  admin sees the §19 UStG notice appear/disappear live as they toggle the
+  field, without this preview needing its own separate mechanism.
+
 ### Company Settings & Live Preview
 
 `company-settings` has a Live Preview button too, like `email-templates`