A 52-year-old Indian woman presents with a 3-week history of painful blisters and erosions in her oral cavity, followed by flaccid blisters on her chest and axillae. On examination, there is extensive oral mucosal involvement with shallow erosions. Gentle pressure on the skin produces separation of the epidermis (positive Nikolsky sign). A skin biopsy shows intraepidermal acantholysis with preserved basal layer attachment (tombstone appearance). Direct immunofluorescence reveals intercellular IgG and C3 deposits in the epidermis. What is the most likely diagnosis?