A 53-year-old woman presents with dyspnea on exertion, orthopnea, paroxysmal nocturnal dyspnea, edema in the legs and feet, and fatigue. She has no history of angina, other signs of coronary artery disease, hypertension, or valvular disease. Echocardiography reveals cardiomegaly, with four-chamber hypertrophy and dilation. Which of the following is the most likely diagnosis?