A 52-year-old woman with bipolar disorder type I has been on lithium carbonate 900 mg daily for 3 years with good mood stabilization. She presents to the emergency department with a 2-day history of coarse tremor, confusion, ataxia, and slurred speech. Her husband reports she had acute gastroenteritis with vomiting and diarrhea for 48 hours, after which she did not reduce her lithium dose. On examination: BP 128/82 mmHg, HR 96/min, temperature 37.2°C, coarse tremor of hands, hyperreflexia, and nystagmus. Serum lithium level is 2.8 mEq/L (normal therapeutic range 0.6–1.2 mEq/L). Serum creatinine 1.4 mg/dL (baseline 0.9 mg/dL), serum sodium 132 mEq/L. What is the most appropriate immediate management?