A 58-year-old woman presents with rapid palpitations and light-headedness that started 6 hours ago. Pulse is irregular at 150 bpm, blood pressure 120/70 mmHg, oxygen saturation 98% on air. ECG confirms new-onset atrial fibrillation with a ventricular rate of 150 bpm. She is clinically stable with no chest pain or heart failure symptoms. What is the most appropriate initial management?