A 52-year-old woman with a history of hypertension and diabetes presents with sudden-onset loss of the upper half of the visual field in both eyes. She denies any eye pain or diplopia. Visual acuity is 6/6 in both eyes. Confrontation visual fields show loss of the superior visual fields bilaterally (above the horizontal meridian), with the inferior fields intact. Fundoscopy reveals flame-shaped hemorrhages and cotton-wool spots in the inferior retina bilaterally. MRI brain is normal. What is the most likely diagnosis?