Add Sumly through Google Tag Manager

Set up Google Tag Manager for Sumly in this project.

Sumly (https://sumly.dev) shows revenue next to the visits and app users it came from. The full guide is https://sumly.dev/install/google-tag-manager; read it if anything below is unclear.

Steps:
1. In Sumly, open Visitors and copy your project's key, the `sl_…` in its line.
2. In Tag Manager, open Tags › New › Tag Configuration › Custom HTML and paste the code into Sumly, with your key in it.
3. Under Triggering, choose Initialization - All Pages, so it loads early on every page. Save the tag.
4. Check it in Preview, then Submit and Publish. Tag Manager changes are not live until published.

Custom HTML: The tag adds Sumly's script itself, with its key on it, rather than leaving it to Tag Manager to copy the attributes across. Keep the double quotes: Tag Manager rewrites single ones.
```
<script>
  (function () {
    var s = document.createElement("script");
    s.defer = true;
    s.src = "https://sumly.dev/s.js";
    s.setAttribute("data-site", "sl_your_key");
    document.head.appendChild(s);
  })();
</script>
```

Keep in mind: Ad blockers that stop Tag Manager stop everything it loads, Sumly too. The line pasted straight into the site, from your own address, gets through more often. Google recommends its built-in tag templates over Custom HTML. There is no template for Sumly, so Custom HTML it is.

How to work:
- Look at how this project is built first, and fit the code to its framework and its way of doing things; the examples above are a starting point, not something to paste as is.
- Steps that happen in a dashboard (Sumly, or Google Tag Manager's own) are mine: tell me exactly what to click, then wait for me.
- Keys the code needs belong in environment variables, never in the code or the repository. Never ask me to paste a secret key into this chat; tell me which variable to set.
- Change only what this needs, show me the changes before you commit, and tell me how to check that it works.
- sl_your_key stands for the project's public key; it is on the Visitors page in Sumly. Ask me for it.