A 70-year-old patient is on long-term digoxin 125 micrograms daily for atrial fibrillation with heart failure. Their renal function has recently declined (eGFR from 60 to 35 mL/min/1.73m²). They present with nausea, vomiting, confusion, and seeing yellowish halos around lights. An ECG shows bradycardia with ventricular bigeminy. What is the most likely diagnosis?