A 52-year-old man from Delhi presents with a 6-month history of progressive darkening and thickening of skin over his neck, axillae, and groin. On examination, he has velvety, hyperpigmented plaques with exaggerated skin markings. He reports a 10 kg unintentional weight loss over the past 3 months and persistent abdominal discomfort. Fasting blood glucose is 248 mg/dL. Abdominal ultrasound reveals a 4 cm heterogeneous mass in the pancreatic head. What is the most likely diagnosis underlying the cutaneous findings?