A 42-year-old woman presents with a 3-day history of severe burning pain and grouped vesicles on an erythematous base along the left T4 dermatome. She denies systemic symptoms and has no prior history of chickenpox. On examination, the vesicles are in various stages of evolution (some crusted, some fluid-filled), confined strictly to one side of the trunk, and do not cross the midline. What is the most likely diagnosis?