Add Custom 404 Page Support

## Feature Description
Currently, the file server responds with a standard "Not Found" status (404) when a requested resource isn't available. To improve the user experience and provide more informative error handling, I propose supporting custom 404 pages.

## Proposed Solution
Add error handling logic to the `createServeDirResponse` function in the server. The new logic should:

1. Check for the existence of a `404.html` file in the root directory when a request for a non-existent resource is made.
2. If present, serve this `404.html` file to the client in place of the standard 404 status.
3. If the `404.html` file doesn't exist, continue returning the standard "Not Found" status.