A 32-year-old woman presents with a 2-day history of dysuria, urinary frequency, and urgency. She is afebrile and has no flank pain. Urinalysis dipstick is positive for leukocytes and nitrites. She has no known drug allergies. According to NICE guidelines for lower UTI in non-pregnant women under 65, what is the recommended first-line empirical antibiotic treatment?