A 28-year-old female presents with fever and night sweats for 6 months. Recent marked abdominal distension is noted. Physical exam: no cardiopulmonary abnormalities, doughy abdominal wall, non-palpable liver/spleen, ascites sign (+). Ascitic fluid analysis: straw-colored, specific gravity 1.02, protein 30g/L, WBC 600×10<sup>6</sup>/L (predominantly monocytes), no pathological cells found. The most likely diagnosis is: