A 24-year-old man with type 1 diabetes presents with polyuria, polydipsia, abdominal pain, and vomiting. His capillary glucose is 26 mmol/L, venous pH is 7.12, bicarbonate 10 mmol/L, and serum ketones are strongly positive. Potassium is 4.8 mmol/L, and blood pressure is 110/70 mm Hg. What is the most important immediate management step?