feat(registration): add email domain whitelist policy

frontend/src/utils/authError.ts

@@ -0,0 +1,25 @@
+interface APIErrorLike {
+  message?: string
+  response?: {
+    data?: {
+      detail?: string
+      message?: string
+    }
+  }
+}
+
+function extractErrorMessage(error: unknown): string {
+  const err = (error || {}) as APIErrorLike
+  return err.response?.data?.detail || err.response?.data?.message || err.message || ''
+}
+
+export function buildAuthErrorMessage(
+  error: unknown,
+  options: {
+    fallback: string
+  }
+): string {
+  const { fallback } = options
+  const message = extractErrorMessage(error)
+  return message || fallback
+}