A 32-year-old man is brought to the emergency department following a motor vehicle collision with blunt chest trauma. On examination, he is dyspneic with a respiratory rate of 28/min, oxygen saturation 88% on room air, and blood pressure 110/70 mmHg. Palpation reveals paradoxical movement of the left anterior chest wall over ribs 4–6. Chest X-ray shows fractures of ribs 4, 5, and 6 on the left side with no pneumothorax or hemothorax. Arterial blood gas shows pH 7.32, PaCO₂ 48 mmHg, PaO₂ 65 mmHg. What is the most appropriate immediate management?