A 34-year-old woman with bipolar I disorder has been stable on lithium 900 mg daily for 2 years. She presents to the emergency department with confusion, coarse tremor, hyperreflexia, and rigidity. Serum lithium level is 2.8 mEq/L (therapeutic range 0.6–1.2 mEq/L). Serum creatinine is 1.8 mg/dL (baseline 0.9 mg/dL), and serum sodium is 128 mEq/L. What is the most appropriate immediate management?