Add bankName back to InvoiceSeller, shown alongside IBAN/BIC

Dropped by accident when bankDetails split into iban/bic — a bank name
has no fixed format to validate, so it stays free text, but it's still
useful context next to the account numbers in the invoice footer.

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

src/correctionInvoicePdf.tsx

@@ -304,7 +304,7 @@ function CorrectionInvoiceDocument({ kind, order, seller }: { kind: CorrectionIn
               invoicePdf.tsx's own comment on this same line. */}
           {(seller.iban || seller.bic) && (
             <Text style={{ marginTop: 4 }}>
-              Bankverbindung: {[seller.iban && `IBAN ${seller.iban}`, seller.bic && `BIC ${seller.bic}`].filter(Boolean).join(" · ")}
+              Bankverbindung: {[seller.bankName, seller.iban && `IBAN ${seller.iban}`, seller.bic && `BIC ${seller.bic}`].filter(Boolean).join(" · ")}
             </Text>
           )}
         </View>

src/invoicePdf.tsx

@@ -340,7 +340,7 @@ export function InvoiceDocument({ order, seller }: { order: InvoiceOrder; seller
               reasons, e.g. a refund) — see this package's README. */}
           {(seller.iban || seller.bic) && (
             <Text style={{ marginTop: 4 }}>
-              Bankverbindung: {[seller.iban && `IBAN ${seller.iban}`, seller.bic && `BIC ${seller.bic}`].filter(Boolean).join(" · ")}
+              Bankverbindung: {[seller.bankName, seller.iban && `IBAN ${seller.iban}`, seller.bic && `BIC ${seller.bic}`].filter(Boolean).join(" · ")}
             </Text>
           )}
         </View>

src/seller.ts

@@ -14,7 +14,9 @@ export type InvoiceSeller = {
   // Structured (Phase 2 of the e-invoicing migration — EN16931 wants
   // discrete PaymentMeans data, not a free-text paragraph a human
   // formatted by hand) — replaces what used to be a single `bankDetails`
-  // textarea field.
+  // textarea field. bankName has no fixed format to validate against
+  // (unlike iban/bic), so it stays plain free text.
+  bankName?: string | null;
   iban?: string | null;
   bic?: string | null;
   // Pflichtangaben in Geschäftsbriefen for registered legal forms (§37a