A 28-year-old woman from Delhi presents with a 6-month history of bloody diarrhoea, tenesmus, and left lower abdominal cramping. On colonoscopy, continuous mucosal inflammation with friability and ulceration is seen starting from the rectum and extending proximally to the sigmoid colon. Biopsy shows crypt distortion, crypt abscess formation, and inflammation confined to the mucosa and submucosa. She has no perianal disease or fistulas. What is the most likely diagnosis?