A 30-year-old female presents with heavy menstrual bleeding for 3 years and skin purpura for one week. Physical examination reveals scattered petechiae on both lower limbs with no sternal tenderness. No hepatosplenomegaly or lymphadenopathy. Lab results: Hb 110g/L, WBC 4.6×10<sup>9</sup>/L, PLT 30×10<sup>9</sup>/L. Bone marrow shows active nucleated cell proliferation with normal granulocytic and erythroid series. Megakaryocytes: 116 (granular type 0.88, naked nuclei 0.12), no platelet-producing megakaryocytes. The most likely diagnosis is: