A 2-year-old boy is brought to the urgent care centre by his parents in the evening. They report he developed a harsh, 'barking' cough overnight, which sounds worse when he cries. He has noisy breathing, particularly when breathing in (inspiratory stridor), and a hoarse voice. He has a mild fever (37.9°C) but seems reasonably alert and playful between coughing fits. What is the most likely diagnosis?