A 30-year-old woman presents with a 6-month history of intermittent diarrhoea, abdominal cramping, and bloating. She reports occasional blood in her stool. Colonoscopy reveals continuous inflammation extending from the rectum up to the splenic flexure, with loss of vascular pattern and ulceration. Biopsies show crypt abscesses and goblet cell depletion. What is the most likely diagnosis?