A 50-year-old man presents with progressive fatigue and pruritus. Examination reveals excoriations but no jaundice. Laboratory tests show ALP 160 U/L (30-110), GGT 200 U/L, ALT 45 U/L, bilirubin 14 µmol/L (normal), and fasting lipid profile with markedly elevated cholesterol. Anti-mitochondrial antibodies are negative; MRCP shows multifocal stricturing and dilatation of intrahepatic and extrahepatic bile ducts. He has long-standing ulcerative colitis. What is the most likely diagnosis?