A 5-year-old girl from Chennai is on day 5 of dengue fever. She now has tachycardia (HR 120/min), systolic BP 90 mmHg (baseline ~105 mmHg), weak pulses, delayed capillary refill (3 seconds), and cold extremities. Haematocrit has risen from 35% to 48%. Platelet count is 45,000/μL. She is restless and has passed minimal urine in the last 4 hours. What is the most appropriate immediate next step?