A 25-year-old woman presents with recurrent lower limb purpura and menorrhagia for 1 year, with no history of medication use prior to illness. Spleen palpable 1 cm below the costal margin. Hemoglobin (Hb) 105 g/L, white blood cell (WBC) count 5.4×10<sup>9</sup>/L, platelets (PLT) 25×10<sup>9</sup>/L. Erythrocyte sedimentation rate (ESR), urinalysis (UA), and liver function tests are normal. No lupus cells found. Increased granular megakaryocytes with otherwise normal findings. The most likely diagnosis is: