A 32-year-old multigravida at 28 weeks of gestation presents with newly diagnosed gestational diabetes. Fasting glucose is 128 mg/dL and 2-hour postprandial glucose is 172 mg/dL. She has a BMI of 31 kg/m² and a strong family history of type 2 diabetes. After 10 days of intensive dietary counseling and exercise, her fasting glucose remains 130 mg/dL. She has no contraindications to any antidiabetic agent. What is the preferred initial pharmacological agent for this patient?