fix(bank): break BankFeedback's shared/api edge for stories (#80)

BankFeedback lived under entities/bank-card/ui but pulled apiErrorCode
from @/shared/api, which dragged the entire api layer (openapi-fetch,
openapi-react-query, @tanstack/react-query, client, problem-details,
resource-query) into any Storybook story that imported a sibling
view, breaking stories-must-not-reach-application-data.

Move the local 'extract code from unknown' helper into the entity
and consume it from BankFeedback. The shared helper stays in
shared/api for the rest of the app.

src/entities/bank-card/model/error-code.ts

@@ -0,0 +1,7 @@
+function errorCode(error: unknown): string | undefined {
+	return typeof error === "object" && error !== null && "code" in error
+		? String(error.code)
+		: undefined;
+}
+
+export { errorCode };