A 52-year-old Indian woman presents with a 3-week history of painful oral erosions and blistering of the oral mucosa. She reports that the blisters rupture easily, leaving painful ulcers. Two weeks later, flaccid blisters appear on the flexural surfaces of the arms and in the axillae. On examination, gentle pressure on apparently normal skin results in separation of the epidermis (positive Nikolsky sign). Oral mucosa shows erosions with surrounding erythema. A skin biopsy shows intraepidermal acantholysis with a "tombstone" appearance of basal cells. Immunofluorescence reveals IgG and C3 deposits in an intercellular pattern. What is the most likely diagnosis?