A 35-year-old woman presents with a 6-month history of intermittent bloody diarrhoea, abdominal cramps, and urgency. Colonoscopy reveals continuous inflammation extending proximally from the rectum to the splenic flexure, with loss of vascular pattern, granularity, and contact bleeding. Biopsies confirm inflammation limited to the mucosa and submucosa with crypt abscesses. What is the most likely diagnosis?