refactor: align frontend ownership boundaries

Close audit gaps across API drift, query policy, UI, and tests.

src/entities/brokerage-account/model/display.ts

@@ -16,12 +16,6 @@ function accountTypeKey(type: BrokerageAccount["type"]) {
 	return `accounts.types.${type}` as const;
 }
 
-function errorCode(error: unknown): string | undefined {
-	return typeof error === "object" && error !== null && "code" in error
-		? String(error.code)
-		: undefined;
-}
-
 function brokerageAccountCards(
 	accounts: BrokerageAccount[],
 ): BrokerageAccountCard[] {
@@ -44,9 +38,4 @@ function brokerageAccountCards(
 	});
 }
 
-export {
-	accountTypeKey,
-	type BrokerageAccountCard,
-	brokerageAccountCards,
-	errorCode,
-};
+export { accountTypeKey, type BrokerageAccountCard, brokerageAccountCards };