A 52-year-old woman on long-term oral corticosteroids (prednisolone 15 mg daily for 8 years due to systemic lupus erythematosus) presents with sudden onset of glare and blurred vision in both eyes over the past 3 months. She reports that her vision is worse in dim light and bright light causes severe discomfort. On slit-lamp examination, fine granular opacities are seen at the posterior pole of the lens, just beneath the posterior capsule. Dilated fundoscopy is otherwise unremarkable. What is the most likely diagnosis?