A 30-year-old woman presents with a 4-month history of bloody diarrhoea, urgency, and tenesmus. She reports passing mucus mixed with stool. Colonoscopy reveals continuous inflammation extending from the rectum to the splenic flexure, with loss of vascular pattern and superficial ulceration. Biopsies show crypt abscesses and goblet cell depletion. What is the most likely diagnosis?