A 32-year-old woman presents with a 5-day history of right eye pain, photophobia, and blurred vision. She reports a preceding upper respiratory tract infection 1 week ago. On examination, visual acuity is 6/9, and slit-lamp shows a dendritic ulcer with terminal bulbs on the cornea staining with fluorescein. Conjunctival injection and preauricular lymphadenopathy are noted. What is the most likely diagnosis and the preferred initial treatment?