A 72-year-old woman is diagnosed with multiple myeloma. Her initial blood tests show: Hb 98 g/L, corrected calcium 3.1 mmol/L (normal 2.2-2.6), creatinine 180 µmol/L (eGFR 28 mL/min/1.73m²). What is the most important initial step in managing her hypercalcaemia, alongside addressing the underlying myeloma?