A 23-year-old woman with type 1 diabetes is found confused and drowsy at home. In the emergency department she is tachypnoeic with Kussmaul respirations, blood pressure 100/60 mmHg and heart rate 118/min. Capillary glucose is 26 mmol/L and venous blood gas confirms metabolic acidosis with ketonaemia. According to diabetic ketoacidosis management principles, what is the most appropriate initial intravenous fluid regimen in the first hour, assuming no contraindications?