A 48-year-old woman with bipolar I disorder has been stable on lithium 750 mg daily for 3 years. She now presents for routine follow-up. Her serum lithium level is 0.8 mEq/L (therapeutic range 0.6–1.2 mEq/L), and she has normal renal and thyroid function. She reports new-onset polyuria and polydipsia. What is the drug of choice to manage her lithium-induced nephrogenic diabetes insipidus?