A 16-year-old boy presents with a 3-month history of progressive pain and swelling in the left femur. He reports fever and weight loss over the past 6 weeks. On examination, there is a firm, warm swelling over the mid-shaft of the femur with restricted hip and knee movements. Laboratory investigations show an elevated ESR (78 mm/hr) and CRP (12 mg/dL). Plain radiographs reveal a mixed lytic and sclerotic lesion in the diaphysis with an onion-skin periosteal reaction. MRI shows a large soft tissue mass with marrow involvement. What is the most likely diagnosis?