fix: auto-select single sub-option and remove unused return_url field

- Auto-select sub-option even when only 1 exists (backend needs suffix)
- Remove return_url from public LandingPaymentMethod (backend doesn't send it)
- Add else branch in auto-select for defensive null reset

src/api/landings.ts

@@ -42,7 +42,6 @@ export interface LandingPaymentMethod {
   min_amount_kopeks: number | null;
   max_amount_kopeks: number | null;
   currency: string | null;
-  return_url: string | null;
   sub_options: LandingPaymentMethodSubOption[] | null;
 }