feat(126): invoice GUS field mapping fix (JDG/KRS heuristic)

MfWhitelistApiClient.lookupByNip() exposes is_jdg/krs from MF Biala Lista.
InvoiceController.nipLookup propagates is_jdg in JSON response.
invoice_form.php JS conditionally targets buyer_name (JDG) or
buyer_company_name (spolka z KRS); second field keeps zamowienie pre-fill.

Fixes apparent field swap on /orders/{id}/invoice/create after GUS lookup
for JDG (sole trader) where MF returns natural person in subject.name.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Modules/Accounting/InvoiceController.php

@@ -55,6 +55,7 @@ final class InvoiceController
                 'country' => $data['country'],
                 'regon' => $data['regon'],
                 'status_vat' => $data['status_vat'],
+                'is_jdg' => (bool) ($data['is_jdg'] ?? false),
             ],
         ]);
     }