Add Sumly to a Squarespace site

Set up Squarespace 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/squarespace; read it if anything below is unclear.

Steps:
1. In Sumly, open Visitors and copy your project's line.
2. In Squarespace, open Website › Website Tools › Code Injection and paste the line into Header.
3. Save. Open the site in a private window and the visit shows on Visitors.

Header: 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>
```

Keep in mind: Code injection needs the Core, Plus or Advanced plan (or some older plans). If the code ever upsets the editor's preview, turn on **Disable Scripts in Preview**; visitors are not affected. A page has its own header box too (page settings › Advanced). The site-wide one makes every page count.

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 Squarespace'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.