A 28-year-old woman presents to the emergency department 15 minutes after eating peanuts at a restaurant. She complains of severe itching of the lips and throat, followed by facial angioedema and wheezing. Her blood pressure is 90/60 mmHg, heart rate 118/min, and respiratory rate 24/min. She has a history of similar reactions to tree nuts. What is the most appropriate immediate next step in management?