A 5-year-old girl presents with sudden onset of multiple bruises on her legs and trunk, appearing over the last 2 days. She had a viral illness about 2 weeks ago but is otherwise well. Examination confirms widespread petechiae and purpura. Full blood count reveals: Hb 120 g/L, WBC 8.0 x 10$^9$/L, Platelets 15 x 10$^9$/L. Coagulation screen is normal. What is the most likely diagnosis?