A 16-year-old boy presents with a 3-month history of progressive pain and swelling over the distal femur. He reports the pain is worse at night and does not respond well to NSAIDs. On examination, there is a firm, fixed mass over the distal femoral metaphysis with warmth and overlying skin erythema. Plain radiographs show a mixed lytic and sclerotic lesion with cortical destruction and a sunburst pattern of new bone formation. The lesion extends into the soft tissues with a Codman triangle visible. What is the most likely diagnosis?