A 42-year-old woman with a history of rheumatic heart disease presents to the emergency department with acute dyspnea, orthopnea, and pink frothy sputum. On examination, she is tachycardic (HR 124/min), tachypneic (RR 28/min), and in atrial fibrillation. Auscultation reveals a loud S1, opening snap, and a diastolic murmur. Chest X-ray shows bilateral pulmonary edema and a straightened left heart border. Transthoracic echocardiography shows a mitral valve area of 0.8 cm² and a left atrial thrombus. What is the immediate management priority?