A 74-year-old woman presents with palpitations and dyspnoea. ECG shows atrial fibrillation with a ventricular rate of 140 beats/min. Blood pressure is 110/70 mmHg, oxygen saturation 98% on room air, and she is otherwise haemodynamically stable. Symptoms started 48 hours ago. Echocardiography 6 months ago showed preserved systolic function. What is the most appropriate initial management strategy?