Phase 2: iban/bic instead of bankDetails, matching the backend collection

CompanySettings type now mirrors the payload repo's split bankDetails ->
iban/bic (see that repo's own commit for the reasoning). No behavior
change here beyond the type/fallback update — the actual footer
rendering lives in @einfach-produktiv/invoicing.

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

app/lib/payload.ts

@@ -737,7 +737,8 @@ export type CompanySettings = {
   sellerEmail: string;
   vatId: string;
   taxRatePercent: number;
-  bankDetails: string | null;
+  iban: string | null;
+  bic: string | null;
 };
 
 // Server-only in practice (only ever called from app/lib/invoiceData.ts),