Build a file explorer in React that uses a flat DOM structure (no nested <div>s for children), but still visually represents hierarchy.

Requirements:

1. Display files and directories in a list, using a flat DOM structure.
2. Directories:
   • Can contain files and subdirectories.
   • Can be expanded or collapsed by clicking.
   • Should be displayed before files and sorted alphabetically.
3. Files:
   • Are leaf nodes, cannot be expanded.
4. Hierarchy must be visually represented by indentation (using styles, not nested DOM).
5. Focus on functionality and flat structure, not styling.