A 7-year-old male presents with a sudden onset of knee, hip and elbow joint pain associated with arthralgia, low to moderate grade fever, a skin rash (described by the mother as "ringworms"), and involuntary purposeless movements. Lab investigations show elevated ESR and C-reactive protein; physical examination reveals pericardial friction rubs, tachycardia, and weak first and second heart sounds, with no murmurs. What is the diagnosis?