A 28-year-old woman presents with a 6-month history of a gradually enlarging lump in the left breast. On examination, a 4 cm firm mass is palpated in the upper outer quadrant with skin dimpling and mild tenderness. Ultrasound reveals a heterogeneous mass with irregular margins, increased vascularity on Doppler, and a maximum dimension of 4.5 cm. Core needle biopsy shows benign stromal and epithelial components with increased cellularity. What is the most likely diagnosis?