A 16-year-old girl presents with mild, intermittent jaundice for the past year. Laboratory studies show: total bilirubin 4.2 mg/dL, direct bilirubin 3.0 mg/dL, normal aminotransferases, and normal alkaline phosphatase. A liver biopsy reveals coarse, dark brown granular pigment in hepatocytes. What is the most likely diagnosis?