A 24-year-old woman with type 1 diabetes is brought to the emergency department with nausea, vomiting, polyuria, and abdominal pain. She stopped taking insulin 2 days ago. Vitals: T 99.1 F, HR 120, BP 100/60 mm Hg, RR 26. Labs: glucose 540 mg/dL, pH 7.18, bicarbonate 9 mEq/L, anion gap 24, positive serum ketones, potassium 4.5 mEq/L. What is the most appropriate initial management step?