A 22-year-old woman with type 1 diabetes presents to the emergency department with 24 hours of vomiting and abdominal pain. She is tachycardic, dehydrated and has Kussmaul respirations. Capillary glucose is 28 mmol/L. Venous blood gas shows pH 7.12, bicarbonate 10 mmol/L, and serum ketones are strongly positive. Potassium is 5.1 mmol/L and creatinine 130 µmol/L. After confirming the diagnosis and drawing initial laboratory tests, what is the most appropriate immediate management step?