A 42-year-old woman with a history of recurrent vomiting due to gastroesophageal reflux disease presents with complaints of muscle weakness and polyuria. She has been taking omeprazole and antacids (aluminum hydroxide) for 6 months. On examination, she appears dehydrated. Vital signs: BP 105/68 mmHg (orthostatic drop of 8 mmHg), HR 88/min. Laboratory findings: - pH: 7.52 - PaCO₂: 48 mmHg - HCO₃⁻: 38 mEq/L - Na⁺: 140 mEq/L - K⁺: 2.8 mEq/L - Cl⁻: 92 mEq/L - Urine Cl⁻: 15 mEq/L (low) - BUN: 32 mg/dL - Creatinine: 1.2 mg/dL What is the most likely acid-base disorder and its underlying mechanism?