Phase 2: structured iban/bic instead of free-text bankDetails

InvoiceSeller.bankDetails becomes iban/bic (EN16931 wants discrete
PaymentMeans data, not a paragraph a human formatted by hand). The
footer's bank-details line also drops the misleading "(für Überweisung)"
qualifier — it was never actually conditional on the order's payment
method (only on whether the field was set at all), just worded as if it
were. Now shown whenever iban or bic is set, regardless of payment
method.

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

src/invoicePdf.tsx

@@ -334,7 +334,15 @@ export function InvoiceDocument({ order, seller }: { order: InvoiceOrder; seller
             {seller.registerCourt && seller.registerNumber ? ` · ${seller.registerCourt} · ${seller.registerNumber}` : ""}
             {seller.managingDirector ? ` · Geschäftsführung: ${seller.managingDirector}` : ""}
           </Text>
-          {seller.bankDetails ? <Text style={{ marginTop: 4 }}>Bankverbindung (für Überweisung): {seller.bankDetails}</Text> : null}
+          {/* Always shown when set, regardless of payment method — not
+              gated on "Überweisung" specifically (a customer who paid by
+              card can still want the seller's bank details for other
+              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(" · ")}
+            </Text>
+          )}
         </View>
       </Page>
     </Document>