A 28-year-old woman with type 1 diabetes mellitus presents to the emergency department with severe dyspnea, fruity-smelling breath, and altered mental status. Arterial blood gas shows pH 7.15, PaCO₂ 22 mmHg, HCO₃⁻ 8 mEq/L, and anion gap 18. Blood glucose is 580 mg/dL. What is the most appropriate immediate next step in management?