How to save to local files from browser?

I created a prototype about a tool that would analyze source code and write metadata about it in files to be read by a web application served locally.

Now I need to specify details about how I'll implement it. The data would need to go both ways, first the backend write to file and the frontend read from file, second the frontend write to file and backend read from file.

My question is about how I do the second part, saving from the browser to local files, knowing that I will be serving the page locally.

I've found this api https://web.dev/native-file-system/ that might be what I need, but it's pretty new and requires permission from the user. I guess I can do it differently because I'll serve it locally.

For more context, here's the video of my prototype