A 25-year-old woman with type 1 diabetes presents with polyuria, vomiting and abdominal pain. Vitals: HR 118 beats/min, BP 98/62 mmHg, RR 26, SpO₂ 99% on room air. Capillary glucose is 27 mmol/L; venous blood gas shows pH 7.14 and bicarbonate 10 mmol/L. After securing the airway and monitoring, what is the most appropriate initial intravenous fluid management?