A 48-year-old woman from Delhi presents with a 2-year history of progressive swelling of the right submandibular gland. She reports recent onset of pain and skin erythema over the swelling. On examination, the mass is firm, fixed to the underlying tissues, and the overlying skin shows signs of ulceration. Intraoral examination reveals displacement of the soft palate to the left. Contrast-enhanced CT shows a heterogeneous mass with areas of necrosis. FNAC reveals sheets of squamoid cells with high mitotic activity and keratinization. What is the most likely diagnosis?