refactor: align frontend ownership boundaries

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

src/shared/api/problem-details.ts

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