A 68-year-old Indian man presents with a 3-week history of tense, fluid-filled blisters on the flexural aspects of his forearms, lower abdomen, and inner thighs. The blisters do not rupture easily and the surrounding skin appears erythematous but not erosive. He denies oral involvement. Examination reveals intact bullae that do not blanch with pressure. Serum testing shows IgG antibodies against the basement membrane zone. A skin biopsy demonstrates subepidermal blistering with intact basal layer and eosinophil-rich infiltrate. What is the most likely diagnosis?