public bool Login(string username, string password)
        {
            var user = _context.Employees.FirstOrDefault(w => w.Username == username && w.Password == password);

            if (user != null)
            {
                User = user;
                IsLoggedIn = true;

                NotifyOfPropertyChange(nameof(User));
                NotifyOfPropertyChange(nameof(IsLoggedIn));

                return true;
            }
            else
            {
                return false;
            }
        }
        public void Logout()
        {
            User = null;
            IsLoggedIn = false;

            NotifyOfPropertyChange(nameof(User));
            NotifyOfPropertyChange(nameof(IsLoggedIn));
        }