A 35-year-old woman from Mumbai presents with right eye pain, swelling, and mild proptosis for 3 days, following an upper respiratory tract infection. On examination, visual acuity is 6/6, there is lid edema and chemosis, but eye movements are full and painless. Fundoscopy is normal with no disc edema. CT orbit shows mild fat stranding around the medial rectus without abscess. Blood culture is negative. What is the most likely diagnosis, and what is the appropriate management?