A 60-year-old woman presents with a 2-week history of fatigue, anorexia, and painless jaundice. On examination, she has scleral icterus and a palpably enlarged, non-tender gallbladder (Courvoisier's sign). Liver function tests show a markedly elevated bilirubin (predominantly conjugated) and alkaline phosphatase, with mildly elevated transaminases. What is the most likely underlying diagnosis?