A 23-year-old woman presents to the emergency department with 24 hours of polyuria, polydipsia, abdominal pain, and vomiting. She has type 1 diabetes treated with basal-bolus insulin. Vitals: heart rate 118 beats/min, blood pressure 98/60 mm Hg, respiratory rate 26/min with Kussmaul respirations. Capillary glucose is 26 mmol/L; venous blood gas shows pH 7.18 and bicarbonate 12 mmol/L. What is the most appropriate initial management step?