A 7-year-old girl is brought to the urgent care clinic by her parents, complaining of progressive left eye discomfort for the past 24 hours. She has also had a runny nose and mild congestion for the past few days. On examination, her left upper and lower eyelids appear swollen and reddened, and she experiences pain with attempts at eye movement. Her left eye is visibly protruding, and she has difficulty following objects with that eye. Her temperature is 38.8 0C. Intraocular pressure is measured at 14 mmHg (normal range 10-21 mmHg). Given these findings, what is the most likely diagnosis and the immediate definitive management strategy to prevent severe complications?