Drop the Netto row for Kleinunternehmer invoices

Netto = Gesamt is structural under §19 UStG (no tax component ever
existed), not the coincidental 0%-this-one-sale case vatExempt orders
are in — showing it right under the §19 notice just duplicated Gesamt.
At the user's own request after reviewing a real invoice.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -79,6 +79,7 @@ Unlike `vatExempt`, this package does **not** expect `unitPrice` to be de-grosse
 
 - Visual PDF (`invoicePdf.tsx`/`correctionInvoicePdf.tsx`): the "enthält X% MwSt." annotation under Gesamt becomes "Gemäß § 19 UStG wird keine Umsatzsteuer berechnet." instead — takes precedence over the `vatExempt` note if (implausibly) both were ever set.
 - E-invoice XML (`buildEInvoiceData.ts`): VAT category `E` + a free-text exemption reason, no VATEX code — see "E-invoicing" above.
+- **No Netto row** (added 2026-07-24, at the user's own request) — every other invoice type still shows Netto directly under the tax note/annotation, including `vatExempt` ones, where Netto = Gesamt is a *coincidence* worth keeping for layout consistency (0% happens to apply to that one sale). For `kleinunternehmer` it isn't a coincidence — §19 UStG means no tax component ever existed on any of this tenant's invoices, so the row would just repeat Gesamt directly under the §19 notice above it instead of adding information.
 
 `InvoiceSeller.vatId` is now optional (`string | null`, was a plain required `string`) for the same reason — a Kleinunternehmer commonly never registers for an USt-IdNr. at all (no intra-EU trade). Every renderer treats a missing value as "omit the USt-IdNr. line/field" rather than printing an empty one: the visual PDF footer drops the whole `· USt-IdNr. …` segment, and `buildEInvoiceData.ts`'s `sellerParty()` omits `cac:PartyTaxScheme` entirely rather than emitting one with a blank `CompanyID`.