A 24-year-old Indian man from Bihar presents with a 3-month history of fever, weight loss, and progressive abdominal distension. Examination reveals massive splenomegaly, hepatomegaly, and generalised lymphadenopathy. Laboratory results show pancytopaenia, elevated total protein with reversed albumin:globulin ratio, and markedly elevated IgG. A splenic aspirate reveals amastigotes within macrophages. What is the most likely diagnosis?