A 23-year-old woman with type 1 diabetes is brought to the emergency department with 24 hours of vomiting and abdominal pain after an influenza-like illness. She has missed several insulin doses. Vitals: temperature 37.8 °C, heart rate 120 beats/min, blood pressure 98/62 mm Hg, respiratory rate 28 breaths/min with deep breathing. Laboratory investigations show capillary glucose 26 mmol/L, serum ketones positive and venous pH 7.15. What is the most appropriate initial management step?