A 71-year-old man presents with a 3-week history of intense pruritus and multiple large blisters on his trunk and proximal extremities. He reports a burning sensation preceding blister formation. He denies oral pain, dysphagia, or eye irritation. Examination shows multiple tense bullae on an erythematous base and several excoriations from scratching. Gentle lateral pressure on normal-appearing perilesional skin does not induce epidermal sloughing. A punch biopsy shows a subepidermal blister with an eosinophil-rich inflammatory infiltrate. Direct immunofluorescence demonstrates linear deposition of IgG and C3 along the basement membrane zone. Which of the following is the most appropriate first-line therapy?