A 41-year-old female patient presents with palpitations, shortness of breath, and bilateral lower extremity edema for 4 months. Physical examination reveals cardiomegaly with bilateral cardiac border expansion, and a grade 3/6 harsh systolic blowing murmur at the apex. Echocardiogram (ECHO) shows generalized cardiac enlargement, predominantly left atrial and ventricular dilation, left ventricular wall thinning, and diffuse hypokinesis. The most likely diagnosis is: