A 23-year-old man with type 1 diabetes presents with abdominal pain and vomiting. He has Kussmaul respiration, smells of ketones, and appears dehydrated. Capillary glucose is 29 mmol/L and blood ketones are 5.5 mmol/L. Venous blood gas shows pH 7.12 and bicarbonate 10 mmol/L. What is the single most important initial treatment?