A 42-year-old woman with no comorbidities presents with a 3-day history of fever, sore throat, and mild cough. RT-PCR confirms SARS-CoV-2 infection. She is fully vaccinated. SpO₂ is 96% on room air, and she has no respiratory distress. She is at high risk for severe disease due to recent chemotherapy for breast cancer. What is the drug of choice for early treatment?