A 52-year-old woman from Mumbai presents with palpitations and dyspnea for 3 days. On examination, heart rate is 78/min and regular. Her 12-lead ECG reveals a PR interval of 0.28 seconds, normal QRS duration of 0.09 seconds, and a QT interval of 0.42 seconds. The PR interval is uniformly prolonged across all leads. There is no evidence of ST elevation or T-wave inversion. What is the most likely diagnosis based on the ECG findings?