A 42-year-old woman presents with intermittent fever for 1 month (peak 40.1°C, nadir 37.5°C), most pronounced in the afternoon, accompanied by arthralgia in limbs and a rash on the face and anterior chest that disappears when afebrile. Joint pain improves when fever subsides. Lab findings: ESR 82mm/h, WBC 16.0×10⁹/L, elevated serum ferritin, ANA(-). The most likely diagnosis is: