A 52-year-old woman from Mumbai undergoes mammography for screening and is found to have a 1.2 cm suspicious mass in the left breast with no palpable lymph nodes on clinical examination. Ultrasound-guided core needle biopsy confirms invasive lobular carcinoma (ILC), grade II, ER/PR positive, HER2 negative. Staging investigations (CT chest/abdomen, bone scan) are negative for distant metastases. Axillary ultrasound shows no suspicious nodes; sentinel lymph node biopsy (SLNB) is planned. Intraoperatively, the sentinel node is identified and removed; frozen section shows no malignancy. What is the most appropriate next step in surgical management?