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 confusion, coarse tremor, ataxia, and diarrhea for the past 8 hours. Her family reports she had gastroenteritis 2 days ago and has been taking over-the-counter ibuprofen 400 mg three times daily for abdominal pain. On examination, she is disoriented to time and place, with 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?