Add Sumly to a Wix site

Set up Wix 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/wix; read it if anything below is unclear.

Steps:
1. In Sumly, open Visitors and copy your project's line.
2. In Wix, open Settings › Custom Code and click + Add Custom Code. Paste the line, give it a name, choose All pages and Load code once, and place it in Head. Click Apply.
3. Open the published site and the visit shows on Visitors.

Custom code: The whole line, as Sumly shows it with your own key.
```
<script defer src="https://sumly.dev/s.js" data-site="sl_your_key"></script>
```

Consent: With Wix's cookie banner: a second piece of custom code, all pages, with its Code Type set to Analytics, so Wix runs it only once the visitor agrees.
```
<script>
  (function tell() { window.sumly ? sumly.consent(true) : setTimeout(tell, 200); })();
</script>
```

Keep in mind: Custom code needs a published site with a connected domain, which on Wix takes a Premium plan. Code added here belongs to the domain: change the site's main domain and add it again. Do not choose **Load code on each new page**: Sumly would start again on every page.

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 Wix'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.