A 28-year-old woman presents to the emergency department with a 3-day history of fever, malaise, and widespread painful blistering of the skin and oral mucosa. She had started taking trimethoprim-sulfamethoxazole for a urinary tract infection 10 days ago. On examination, she has extensive erythematous macules and target lesions involving >30% of her body surface area, with painful erosions of the lips, tongue, and pharynx. Nikolsky sign is positive. Blood pressure is 100/65 mmHg, and she appears unwell. What is the most appropriate immediate management?