A 30-year-old woman complains of mild burning sensation in her lower lip and its dryness. She peels skin scales off with her teeth. She has been presenting with this condition for 10 years. On examination the skin scales are gray and located on the lip from the Klein’s line to the center of the vermillion border from angle to angle of the mouth. The scales are firmly attached in the center and are loose on the periphery. Their forcible removal does not result in erosions. What is the most likely diagnosis?