A 58-year-old woman with alcoholic cirrhosis (Child–Pugh B) presents with progressive abdominal distension, weight gain of 3 kg in 2 weeks, and ankle oedema. On examination, she has ascites, hepatomegaly, and no signs of spontaneous bacterial peritonitis (SBP). Serum sodium is 128 mEq/L, and her urine sodium is 5 mEq/L. She is currently on spironolactone 100 mg daily. What is the most appropriate next step in management?