A 45-year-old woman with known Addison's disease (primary adrenal insufficiency) presents to A&E with vomiting, abdominal pain, and profound weakness. She ran out of her hydrocortisone tablets 2 days ago. On examination, she is confused, BP is 80/50 mmHg, and HR is 120 bpm. Bloods show Na+ 125 mmol/L, K+ 5.8 mmol/L, Glucose 2.5 mmol/L. What is the most critical immediate intervention?