A 52-year-old woman with bipolar disorder has been on lithium carbonate 900 mg daily for 3 years with good mood stabilization. She presents to the emergency department with confusion, coarse tremor, ataxia, and slurred speech for 2 days. Her family reports she had gastroenteritis with vomiting and diarrhea for 3 days and continued taking her lithium without adjustment. On examination: BP 130/80 mmHg, HR 92/min, temperature 37.2°C, generalized hyperreflexia, and nystagmus. Serum lithium level is 2.8 mEq/L (normal therapeutic range 0.6–1.2 mEq/L). Serum creatinine is 1.4 mg/dL (baseline 0.9 mg/dL). What is the most appropriate immediate management?