A 27-year-old woman presents to her GP with a 3-month history of increased bruising, particularly on her legs, without significant trauma. She also reports her periods have become much heavier recently. She denies any other bleeding or systemic symptoms. A full blood count reveals: Hb 125 g/L (115-165), WCC 7.0 x10^9/L (4.0-11.0), Platelet count 42 x10^9/L (150-400). Peripheral blood film confirms isolated thrombocytopenia with normal red cell and white cell morphology. Coagulation screen (PT, APTT) is normal. What is the most likely diagnosis?