A 72-year-old woman from Mumbai with a history of hypertension and diabetes mellitus type 2 presents with palpitations and mild dyspnea for 3 days. She denies chest pain. Vital signs: BP 138/88 mmHg, HR 110 bpm (irregular), RR 20/min. On auscultation, the first heart sound is variable in intensity. Her 12-lead ECG shows an irregular rhythm with no discernible P waves, and the ventricular rate varies between 90 and 130 bpm. The QRS duration is 0.08 seconds. What is the most likely diagnosis?