A 3-year-old boy is brought to the GP with a 3-day history of fever, cough, and runny nose. Today, his mother noticed he has noisy breathing, particularly on inspiration. On examination, he is alert, temperature is 37.9°C, RR 30/min, HR 110/min, Sats 96% on air. Auscultation reveals inspiratory stridor but no wheeze or crackles. What is the most likely diagnosis?