improvement: error handeling structure changed and unified all across the app. user no longer sees inappropriate messages.

frontend/src/types/api.ts

@@ -1,5 +1,66 @@
+export interface ApiErrorDetail {
+  field: string;
+  code: string;
+}
+
 export interface ApiError {
   statusCode: number;
-  message: string | string[];
-  error?: string;
+  code: string;
+  details?: ApiErrorDetail[];
+}
+
+export interface ApiErrorResponse {
+  success: false;
+  error: {
+    code: string;
+    statusCode: number;
+    details?: ApiErrorDetail[];
+  };
+}
+
+export function isApiError(value: unknown): value is ApiError {
+  return (
+    typeof value === 'object' &&
+    value !== null &&
+    'code' in value &&
+    typeof (value as ApiError).code === 'string' &&
+    'statusCode' in value &&
+    typeof (value as ApiError).statusCode === 'number'
+  );
+}
+
+export function asApiError(value: unknown): ApiError | null {
+  if (isApiError(value)) {
+    return value;
+  }
+
+  if (
+    typeof value === 'object' &&
+    value !== null &&
+    'response' in value &&
+    typeof (value as { response?: { data?: ApiErrorResponse } }).response?.data === 'object'
+  ) {
+    const payload = (value as { response: { data?: ApiErrorResponse } }).response.data;
+    if (payload?.success === false && payload.error?.code) {
+      return {
+        statusCode: payload.error.statusCode,
+        code: payload.error.code,
+        details: payload.error.details,
+      };
+    }
+  }
+
+  return null;
+}
+
+/** @deprecated Use ApiError.code — kept for transitional parsing only. */
+export function legacyStatusCode(value: unknown): number | undefined {
+  if (isApiError(value)) {
+    return value.statusCode;
+  }
+  if (typeof value === 'object' && value !== null && 'statusCode' in value) {
+    const statusCode = (value as { statusCode?: unknown }).statusCode;
+    return typeof statusCode === 'number' ? statusCode : undefined;
+  }
+  return undefined;
 }