A 70-year-old woman presents with sudden onset of severe pain and redness in her right eye, associated with blurred vision and seeing haloes around lights. On examination, the eye is injected, the cornea appears hazy, and the pupil is mid-dilated and poorly reactive to light. Intraocular pressure is measured at 55 mmHg (normal range ~10-21 mmHg). What is the most likely diagnosis?