A 58-year-old man with poorly controlled diabetes mellitus (HbA1c 11.2%) presents with a 5-day history of facial swelling, black necrotic eschar on the hard palate, and fever (38.9°C). He reports rhinosinusitis symptoms and blurred vision in the left eye. On examination, there is proptosis and ophthalmoplegia. Blood glucose is 380 mg/dL, and serum pH is 7.28. A biopsy of the palatal lesion shows broad, non-septate hyphae with right-angle branching. What is the most likely diagnosis?