A 35-year-old man presents 4 hours after a fall onto a flexed wrist. Examination reveals a 'garden spade' deformity with volar angulation and displacement of the distal radius. Neurovascular examination is normal. X-rays confirm a Smith fracture (volar Barton type) with intra-articular extension and 4 mm step-off at the radiocarpal joint. After initial analgesia, what is the most appropriate next step in management?