A 27-year-old female presents with low-grade fever, alternating diarrhea and constipation, abdominal distension for 6 months, and gradually increasing abdominal girth for 1 month. Physical exam: distended abdomen, doughy abdomen, mild tenderness and rebound tenderness, liver and spleen not palpable below the costal margin, shifting dullness (+). Ascites WBCs 450×10⁹/L (PMNs 8%, monocytes 92%), total protein in ascites 55g/L, albumin 43g/L (serum albumin 45g/L), adenosine deaminase (ADA) 75 U/L. The most likely diagnosis is: