A 32-year-old woman from Delhi presents to the emergency department with a 6-month history of recurrent, unexpected episodes of intense fear lasting 15–20 minutes. During these episodes, she experiences palpitations, diaphoresis, trembling, chest discomfort, and a fear of dying or losing control. Between episodes, she worries constantly about having another attack and has begun avoiding crowded places, public transport, and shopping malls. She denies any recent stressors, substance use, or medical illness. Vital signs are normal; physical examination and ECG are unremarkable. What is the most likely diagnosis?