A 62-year-old man presents with a 3-month history of painless jaundice, pale stools, and dark urine. He reports progressive weight loss of 8 kg over 2 months and mild epigastric discomfort. On examination, the abdomen is soft with a palpable gallbladder. Laboratory investigations show total bilirubin 12 mg/dL (direct 10 mg/dL), alkaline phosphatase 420 U/L, and ALT 85 U/L. Abdominal ultrasound reveals dilated intrahepatic bile ducts and a hypoechoic lesion in the pancreatic head measuring 3 cm. CT abdomen with contrast shows a hypodense mass in the pancreatic head with involvement of the superior mesenteric vein. What is the most appropriate next step in management?