A 24-year-old woman with type 1 diabetes presents with nausea, polyuria, and abdominal pain. Vitals: HR 118, BP 98/62, RR 26. Labs: glucose 420 mg/dL, pH 7.08, bicarbonate 8 mEq/L, anion gap 24, serum K⁺ 2.9 mEq/L, creatinine 1.0 mg/dL. After 2 L of isotonic crystalloid, what is the next best step?