Spicier Curry Chicken Salad

Name: Spicier Curry Chicken Salad
Ingredients: 1 pound boneless, skinless chicken breasts, cooked and chopped into bite-size pieces; 2 tablespoons finely diced shallots; 1/4 cup diced celery, plus 1 tablespoon chopped leaves; 1/2 cup halved red grapes; 1 tablespoon lemon juice, or to taste; 2 tablespoons mayonnaise; 1 tablespoon curry powder; 1 pinch cayenne pepper; 1 pinch sugar; Kosher salt and freshly ground black pepper, to taste; White sandwich bread or Little Gem lettuce cups, for serving