A 62-year-old woman with a 40 pack-year smoking history presents with progressive weakness, hyponatremia (Na 118 mEq/L), and elevated urine osmolality (650 mOsm/kg). Chest X-ray reveals a central lung mass. Bronchoscopy confirms small cell lung cancer. CT staging shows no distant metastases. Laboratory evaluation confirms syndrome of inappropriate antidiuretic hormone (SIADH) secondary to SCLC. What is the most appropriate immediate management?