A 28-year-old woman presents to the emergency department with fever (38.5°C), severe oral pain, and difficulty swallowing for 3 days. She reports a maculopapular rash that started on her trunk 2 days ago and has now spread to her face, neck, and proximal limbs. On examination, she has extensive mucosal erosions involving the lips, buccal mucosa, and pharynx. Her skin shows targetoid lesions with central blistering and surrounding erythema. She was started on amoxicillin-clavulanate for a respiratory tract infection 5 days ago. Laboratory investigations show WBC 8,500/μL, hemoglobin 12.5 g/dL, and platelet count 180,000/μL. Skin biopsy shows full-thickness epidermal necrosis with minimal dermal inflammation. What is the most likely diagnosis?