A 52-year-old Indian woman presents with a 3-week history of painful blisters and erosions on her buccal mucosa, palate, and gingiva. She reports that the oral lesions preceded skin involvement by 2 weeks. On examination, she has flaccid blisters on the flexural areas of the neck, axillae, and groin. Gentle pressure on an intact blister causes the blister roof to separate from the base (positive Nikolsky sign). Histopathology 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?