export function formatApiErrorMessage(err: unknown, fallback: string): string {
  if (err && typeof err === 'object' && 'message' in err) {
    const m = (err as { message: unknown }).message;
    if (Array.isArray(m)) {
      return m.filter(Boolean).join(', ');
    }
    if (typeof m === 'string' && m.trim()) {
      return m;
    }
  }
  return fallback;
}