A 35-year-old man with a history of tuberculosis is admitted to the ward with fever, widespread blistering rash, and oral ulceration that began 10 days after starting anti-TB therapy (isoniazid, rifampicin, pyrazinamide, ethambutol). Skin examination reveals flaccid blisters, erosions on >40% of the body surface area, and a positive Nikolsky sign. Histopathology shows subepidermal blister formation with full-thickness epidermal necrosis. He has mild hypotension and tachycardia. What is the most appropriate immediate management?