A 28-year-old non-pregnant woman presents with dysuria and urinary frequency for 24 hours. She is otherwise healthy with no urinary tract abnormalities. She is afebrile and examination is unremarkable. Urinalysis shows leukocytes and nitrites. According to Therapeutic Guidelines and the national primary care antibiotic summary, what is the preferred first-line empirical antibiotic for acute uncomplicated cystitis in this setting?