A 72-year-old woman from Mumbai with a 10-year history of hypertension and diabetes mellitus presents with sudden loss of vision in the left eye. She describes a 'shadow' or 'curtain' that started at the top of her visual field and descended over 30 minutes. Visual acuity in the left eye is counting fingers at 1 meter. The left pupil is sluggish but reactive. Fundoscopy shows a pale, edematous optic disc with retinal whitening and a cherry-red spot at the macula. What is the most likely diagnosis?