Amy DeStefano was a single mom juggling family life and a full-time job three winters ago when she caught what at first was just a persistent February cold. Then it spiraled into pneumonia and spread to her heart. By last January, at age 39, DeStefano was on the waiting list for a heart transplant at Massachusetts General Hospital, and late last month, the Portsmouth, NH, social worker became the first patient in New England to receive an experimental “beating heart’’ transplant.