A 35-year-old man is brought to the emergency department with a 2-week history of progressive headache, morning vomiting, and blurred vision. He denies eye pain or photophobia. On examination, visual acuity is 6/9 bilaterally. Fundoscopy shows bilateral disc swelling with blurred margins, obscuration of vessels at the disc, and retinal folds (Paton's lines) around the disc. Visual fields show enlargement of the blind spot. Lumbar puncture shows opening pressure of 35 cm H₂O. MRI brain reveals a 3 cm pineal region mass compressing the third ventricle. What is the most likely diagnosis?