A 52-year-old woman presents with a 3-month history of painful erosions on her buccal mucosa and gingiva. On examination, she has white reticular patches on the dorsal surface of the tongue and erosive lesions on the gums. She also reports pruritic purple papules on her flexural surfaces and inner wrists. Dermoscopy of the skin lesions shows Wickham's striae. She denies oral ulcers elsewhere and has no systemic symptoms. What is the most likely diagnosis?