link to the code from the website footer

Please link to the [code](https://github.com/Two9A/ThreadTree) from the website footer, so that visitors can easily file issues or contribute code.

I would suggest something like "Thread Tree [service](https://threadtree.xyz/) and [code](https://github.com/Two9A/ThreadTree) by Imran Nazar" with links to the front page on "service" and to the GitHub repo on "code".