A 28-year-old man with type 1 diabetes mellitus presents to the emergency department with a 3-day history of polyuria, polydipsia, and progressive lethargy. On examination, he is drowsy, respiratory rate 32/min with deep breathing, and blood pressure 110/70 mmHg. Capillary blood glucose is 520 mg/dL. Arterial blood gas shows pH 7.18, HCO₃⁻ 12 mEq/L, and PaCO₂ 28 mmHg. Serum sodium is 128 mEq/L (corrected). Urine ketones are 4+. What is the most appropriate immediate next step in management?