A 38-year-old woman from Bangalore presents with a 3-week history of progressive right eye pain, redness, and blurred vision. She is an avid contact lens wearer (daily wear, cleaned with tap water). On slit-lamp examination, there is a central corneal ulcer with an irregular, feathery border and stromal infiltration. The cornea shows a characteristic 'map-like' appearance with satellite lesions. Corneal sensation is markedly reduced. Gram stain and bacterial culture are negative. KOH mount of corneal scrapings shows trophozoites and cysts. What is the most appropriate initial management?