Spaghetti with Chicken and Peppers

Name: Spaghetti with Chicken and Peppers
Ingredients: Coarse salt; 1 pound whole-wheat spaghetti; 1 1/2 cups canned reduced-sodium chicken broth; 3 garlic cloves, minced; 1/2 teaspoon red-pepper flakes; 1/4 cup fresh lime juice (from 2 limes); 3 tablespoons ketchup; 2 tablespoons soy sauce; 2 teaspoons sugar; 10 ounces cooked chicken breast, shredded (2 cups); 2 yellow bell peppers (ribs and seeds removed), thinly sliced; 1/2 cup chopped fresh cilantro; 1 head napa cabbage, cored and shredded, for serving (optional)