A 38-year-old woman with poorly controlled diabetes mellitus presents with a 10-day history of progressive corneal opacity and pain in the right eye. She recalls a minor corneal abrasion from a contact lens 3 weeks ago. Slit-lamp examination shows a corneal ulcer with feathery, ill-defined borders, stromal haze, and satellite lesions. Potassium hydroxide (KOH) mount of corneal scrapings reveals branching, septate hyphae. Visual acuity is counting fingers. What is the most appropriate initial management?