fix: Add User-Agent header to fix CDN blockage

[makisukurisu]

This pull request addresses the issue #150, by replacing a default User-Agent header from FlutterDownloader to a custom one.

This effect can also be achieved, as described in #150, by simply removing this header altogether, however I have not tried to do this.

I can provide a screenshot of the app interface with working downloads, as well as console output:

Feel free to change the user agent, discard other alterations, or just ditch this change in favor of some other edit.

[makisukurisu]

Note: If you try to restart older download tasks, they will still fail, I assume because headers are also saved in the database, so retrying the task sends the same request.

[nullxception]

Thank you so much 😭