A 50-year-old man complains of gum discoloration, pain when eating spicy food, weakness, and irritability. He notes that periodically spots and vesicles appear on his skin. For a long time he has been working in the galvanic workshop of a factory. On examination the patient is pale, he has a gray-black border on his gums, and on his oral mucosa there are single erosions that are painful on palpation. What is the most likely provisional diagnosis?