A 28-year-old woman presents to the emergency department with acute-onset dyspnea, chest tightness, and wheezing for 2 hours. She has a 10-year history of asthma, well-controlled on a low-dose inhaled corticosteroid (ICS). Today she was exposed to her friend's cat (a known trigger). On examination: respiratory rate 28/min, peak expiratory flow (PEF) 60% of predicted, widespread end-expiratory wheeze bilaterally, speaking in short phrases. Oxygen saturation is 92% on room air. Chest X-ray shows hyperinflation with no infiltrates. Which is the most appropriate immediate management?