A 52-year-old woman from Delhi presents with a 3-month history of painful erosions on her buccal mucosa and gingiva. On examination, she has a reticular pattern of white lines on the oral mucosa (Wickham's striae) along with erosive lesions. She also reports a pruritic rash on her flexural wrists and lower legs with shiny, flat-topped purple papules. Dermoscopy of the skin lesions shows a characteristic reticular pattern with white dots and lines. What is the most likely diagnosis?