A 52-year-old woman with gallstones presents with acute onset epigastric pain, vomiting, and elevated pancreatic enzymes (amylase 800 U/L, lipase 1600 U/L). She has received IV fluids for 48 hours and is clinically improving. Repeat laboratory tests show: WBC 12,000/µL, creatinine 1.1 mg/dL, bilirubin 2.8 mg/dL, ALT 320 U/L, AST 280 U/L. Abdominal ultrasound confirms gallstones and a dilated common bile duct (8 mm). What is the most appropriate next step in management?