A 55-year-old female patient complains of pain during eating food, erosions covering oral mucosa. The disease onset was more than one month ago. Objectively: the mucosa of the gums, soft palate and dental arches is unchanged, but has large bright-red erosions. The mucosa is easily damaged and exfoliates when rubbed. The pain is slight. Impression smears contain Tzanck cells. What is the most probable diagnosis?