A 4-year-old girl is brought to the pediatric urgent care by her parents due to acutely worsening respiratory distress. Her symptoms started approximately four days prior with typical upper respiratory infection signs, but over the last 12-18 hours, she developed a sudden onset of high-grade fever peaking at 39.8°C, a harsh, productive cough, and progressive difficulty breathing. Her parents report she has been increasingly irritable and lethargic. On arrival, she appears distressed and is tripodding. Auscultation reveals coarse breath sounds with prominent inspiratory and expiratory stridor. Her voice is muffled and she has a minimal, non-barking cough. Oral examination shows no significant drooling or pharyngeal swelling. Capillary refill is sluggish at 3 seconds, and her skin is mottled. Lateral and anterior-posterior neck radiographs performed in the emergency department, after transfer from urgent care, were unremarkable for epiglottic swelling or subglottic narrowing. Given the rapid deterioration and clinical presentation, what is the most likely diagnosis and the immediate definitive intervention required?