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 the surrounding skin appears normal. She denies oral involvement. On examination, a positive Nikolsky sign is absent. Laboratory investigations show normal complete blood count and liver function tests. Direct immunofluorescence (DIF) of perilesional skin reveals linear IgG and C3 deposits along the basement membrane zone. What is the most likely diagnosis?