A 27-year-old woman develops generalised urticaria, lip swelling, hoarseness, wheeze and profound hypotension within minutes of eating a peanut-containing dessert at a restaurant. She is brought to the emergency department. Her observations are: pulse 132 bpm, blood pressure 78/42 mmHg, respiratory rate 30/min, oxygen saturation 88% on air. What is the single most appropriate immediate treatment?