A 52-year-old woman from Mumbai presents with a 3-month history of progressive right upper quadrant pain, weight loss of 4 kg, and low-grade fever. On examination, she is icteric with a palpable gallbladder. Laboratory investigations show total bilirubin 4.2 mg/dL, direct bilirubin 3.8 mg/dL, and alkaline phosphatase 320 IU/L. Transabdominal ultrasound reveals a thick-walled gallbladder with a large stone impacted at the neck, dilated intrahepatic bile ducts, and a hypoechoic mass in the gallbladder fundus that does not move with position change. What is the most likely diagnosis?