A 24-year-old woman presents to her GP in Melbourne with 2 days of dysuria, urinary frequency and suprapubic discomfort. She is afebrile, not pregnant and has no flank pain. Dipstick urinalysis shows nitrites and leucocytes. Her eGFR is 80 mL/min/1.73 m². According to current Therapeutic Guidelines, what is the most appropriate first-line empirical antibiotic?