A 32-year-old woman presents with a 6-month history of intermittent bloody diarrhoea, abdominal cramps, and urgency. She also reports feeling fatigued and has lost 3kg in weight. A colonoscopy reveals continuous inflammation extending from the rectum to the splenic flexure, with loss of vascular pattern, friability, and pseudopolyps. Biopsies confirm active chronic colitis. What is the most likely diagnosis?