A 22-year-old woman presents to the breast clinic with a 2-month history of a painless lump in the upper outer quadrant of her left breast. On examination, a firm, mobile, well-circumscribed mass measuring 1.5 cm is palpated. The mass is non-tender and moves freely with skin and underlying tissues. There is no nipple discharge, skin dimpling, or axillary lymphadenopathy. Ultrasound shows a hypoechoic, homogeneous lesion with well-defined margins. What is the most likely diagnosis?