issues/27: add formatApiError util and unit tests

src/utils/__test__/formatApiError.test.js

@@ -0,0 +1,19 @@
+import { formatApiError } from '../formatApiError';
+
+describe('formatApiError', () => {
+  it('returns string errors as-is', () => {
+    expect(formatApiError('bad')).toBe('bad');
+  });
+
+  it('reads RTK-style data.message', () => {
+    expect(formatApiError({ data: { message: 'Validation failed' } })).toBe('Validation failed');
+  });
+
+  it('reads axios-style response.data.error', () => {
+    expect(formatApiError({ response: { data: { error: 'Unauthorized' } } })).toBe('Unauthorized');
+  });
+
+  it('falls back to error.message', () => {
+    expect(formatApiError({ message: 'Network' })).toBe('Network');
+  });
+});

src/utils/formatApiError.js

@@ -0,0 +1,27 @@
+/**
+ * Normalize API error payload for UI (RTK Query / axios).
+ * @param {unknown} error
+ * @returns {string}
+ */
+export function formatApiError(error) {
+  if (!error) {
+    return 'Unknown error';
+  }
+  if (typeof error === 'string') {
+    return error;
+  }
+  const data = error?.data ?? error?.response?.data;
+  if (typeof data === 'string') {
+    return data;
+  }
+  if (data?.message) {
+    return String(data.message);
+  }
+  if (data?.error) {
+    return String(data.error);
+  }
+  if (error?.message) {
+    return String(error.message);
+  }
+  return 'Request failed';
+}