A 23-year-old woman with type 1 diabetes presents with abdominal pain, vomiting and polyuria. Capillary glucose is 27 mmol/L and urine ketones are strongly positive. Arterial blood gas shows pH 7.10 and bicarbonate 10 mmol/L. She is drowsy but rousable and systolic BP is 102 mmHg. After ABCDE assessment, what is the most appropriate initial fluid management?