fix: handle Pydantic validation errors in notify + nullify empty optional fields

- getApiErrorMessage now formats Pydantic [{loc, msg}] arrays as readable strings
- LandingPaymentMethod description/icon_url nullable
- Send null instead of empty string for optional payment method fields

src/api/landings.ts

@@ -30,8 +30,8 @@ export interface LandingTariff {
 export interface LandingPaymentMethod {
   method_id: string;
   display_name: string;
-  description: string;
-  icon_url: string;
+  description: string | null;
+  icon_url: string | null;
   sort_order: number;
 }