A newborn baby girl, born at term via normal vaginal delivery, is noted to have respiratory distress shortly after birth. Examination reveals tachypnoea (RR 70/min), nasal flaring, grunting, and mild intercostal recession. Oxygen saturation is 92% in air. Chest X-ray shows hyperinflation with fluid in the horizontal fissure. Symptoms typically improve within 24-72 hours. What is the most likely diagnosis?