A 72-year-old woman with chronic heart failure (ejection fraction 35%) and hypertension presents with nausea, headache, and mild confusion for 2 days. She denies polydipsia or polyuria. On examination, she is euvolemic with no peripheral edema. Blood pressure is 135/88 mmHg. Laboratory results: Na⁺ 124 mEq/L, K⁺ 4.2 mEq/L, creatinine 1.2 mg/dL (baseline 1.0 mg/dL), BUN 28 mg/dL, urine osmolality 520 mOsm/kg, serum osmolality 256 mOsm/kg, and urine Na⁺ 18 mEq/L. Which of the following is the most likely diagnosis?