refactor: enforce enum machine codes #18

Use owner enums for known codes; preserve wire values and unknown fallbacks.

Verified npm ci and verify: 379 tests, 5 lint checks, app and Storybook builds.

src/shared/api/problem-details.test.ts

@@ -0,0 +1,13 @@
+import { expect, it } from "vitest";
+
+import { ApiErrorCode, apiErrorCode, isApiErrorCode } from "./problem-details";
+
+it("matches known codes and preserves unknown wire codes for fallback handling", () => {
+	expect(isApiErrorCode(apiErrorCode({ code: "UNAUTHORIZED" }), ApiErrorCode.Unauthorized)).toBe(true);
+	const unknownCode = apiErrorCode({ code: "FUTURE_PROVIDER_ERROR" });
+	expect(unknownCode).toBe("FUTURE_PROVIDER_ERROR");
+	expect(isApiErrorCode(unknownCode, ApiErrorCode.Unauthorized)).toBe(false);
+	expect(isApiErrorCode(undefined, ApiErrorCode.Unauthorized)).toBe(false);
+	expect(apiErrorCode(null)).toBeUndefined();
+	expect(apiErrorCode({})).toBeUndefined();
+});