Window Trial of Intratumoral (IT) Neoadjuvant Fluorescently Labelled Nivolumab (nivo800) in Head and Neck Squamous Cell Carcinoma (HNSCC)

The goal of this clinical trial is to learn whether intratumoral (injected directly into the tumor) fluorescently labeled nivolumab (nivo800) is safe for adults with head and neck squamous cell carcinoma (HNSCC) who are scheduled to undergo surgery. Researchers also want to learn how well nivo800 travels to nearby lymph nodes and whether it can be used to study immune responses in and around the tumor. The main questions it aims to answer are: * Is intratumoral nivo800 safe and well tolerated in patients with HNSCC? * How much of the injected nivo800 reaches and remains in nearby lymph nodes, and how is drug delivery related to immune and tumor responses? Participants will: * Receive a first injection of nivo800 directly into the tumor approximately 14-21 days before surgery. * Receive a second injection of nivo800 directly into the tumor approximately 1-3 days before surgery. * Have physical exams, vital signs assessments, blood tests, and safety monitoring during the study. * Undergo their planned standard-of-care surgery, during which tumor and lymph node tissues will be collected for research analyses. * Provide blood samples and complete follow-up assessments to evaluate side effects and study drug distribution.