A 25-year-old man with type 1 diabetes presents with polyuria, vomiting, and abdominal pain. Vitals show heart rate 120 beats per minute, blood pressure 102/64 mm Hg, and respiratory rate 28 with Kussmaul respirations. Capillary glucose is 24 mmol/L, venous pH 7.12, bicarbonate 10 mmol/L, and serum ketones are elevated. What is the most important initial management step in diabetic ketoacidosis?