A 68-year-old Indian woman presents with a 3-week history of tense, fluid-filled blisters on the flexural aspects of both forearms and inner thighs. The blisters do not rupture easily and are preceded by urticarial lesions. She denies oral involvement. Physical examination reveals intact bullae with clear fluid and erythematous urticarial plaques at the periphery. A skin biopsy shows subepidermal blister formation with eosinophil-rich infiltrate. Direct immunofluorescence reveals linear IgG and C3 deposits along the basement membrane zone. What is the most likely diagnosis?