A 62-year-old woman presents with sudden onset vertigo, nausea, and vomiting, lasting continuously for the past 2 days. She is unsteady on her feet but has no focal neurological deficits, headache, or hearing loss. Examination shows horizontal nystagmus that is suppressed by visual fixation. Head impulse test is positive (corrective saccade when head turned rapidly towards the affected side). What is the most likely diagnosis?