A 52-year-old postmenopausal woman from Delhi presents with progressive abdominal distension and mild abdominal pain for 3 months. On examination, she has ascites and a palpable abdominal mass. Serum CA-125 is markedly elevated at 850 U/mL. Ultrasound shows a large complex ovarian mass with solid and cystic components, and free fluid in the pelvis. CT abdomen reveals omental caking and peritoneal nodules. What is the most likely diagnosis?