A 35-year-old man from rural Uttar Pradesh presents with recurrent episodes of hemoptysis and exertional dyspnea for 2 years. He has a history of acute rheumatic fever at age 12. On examination, he has an irregular pulse, a diastolic murmur at the apex, and signs of pulmonary edema. Chest X-ray shows pulmonary congestion and a double contour of the right heart border. Echocardiography demonstrates a dilated left atrium (LA diameter 5.5 cm), spontaneous echo contrast ("smoke"), and a thrombus in the left atrial appendage (LAA). What is the most important immediate intervention to prevent a life-threatening complication?