refactor(downloads): extract DeleteCategoryController with clean URL route
Replaces legacy ?act=folder_delete include with DeleteCategoryController
at /downloads/categories/{id}/delete. Extracts DeleteCategoryUseCase
for domain logic. Adds folder name to the confirmation page.
at /downloads/categories/{id}/delete. Extracts DeleteCategoryUseCase
for domain logic. Adds folder name to the confirmation page.