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/company-settings-preview/page.tsx

@@ -22,7 +22,8 @@ const FALLBACK: CompanySettings = {
   sellerEmail: "",
   vatId: "",
   taxRatePercent: 19,
-  bankDetails: null,
+  iban: null,
+  bic: null,
 };
 
 // Entered exclusively via CompanySettings.ts's admin.livePreview.url (a