A 52-year-old Indian man presents with progressive darkening and thickening of skin over his neck, axillae, and groin for the past 8 months. He reports associated weight loss of 6 kg over 3 months and mild pruritus. On examination, the affected areas show velvety, hyperpigmented plaques with exaggerated skin markings. His fasting blood glucose is 186 mg/dL, and HbA1c is 9.2%. Abdominal examination reveals a palpable epigastric mass. What is the most likely underlying diagnosis?