A 28-year-old woman from Delhi presents with bloody diarrhea, fever (39.2°C), and severe abdominal pain for 3 days. Stool culture confirms Shigella sonnei. On examination, she has signs of severe dehydration, tachycardia (HR 110/min), and rebound tenderness. Abdominal X-ray shows a dilated colon (8 cm diameter). What is the most appropriate immediate next step in management?