A newborn baby girl, born at term via normal vaginal delivery, is noted to have rapid breathing (tachypnoea) shortly after birth. Her respiratory rate is 75/min, with mild subcostal recession and nasal flaring. She is centrally pink with oxygen saturation of 94% in air. Chest X-ray shows hyperinflation, fluid in the horizontal fissure, and prominent vascular markings. Symptoms resolve spontaneously within 48 hours. What is the most likely diagnosis?