A 32-year-old male labourer presents to the emergency department following a fall on his outstretched left hand while working at height. He reports severe pain in the proximal forearm and inability to supinate the forearm. On examination, there is swelling over the proximal third of the forearm with a palpable anterior angulation. X-ray of the left forearm shows a fracture of the proximal third of the ulna with anterior displacement of the radial head. What is the most likely diagnosis?