Add Sumly to a Shopify store

Set up Shopify 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/shopify; read it if anything below is unclear.

Steps:
1. In Sumly, open Visitors and copy your project's line.
2. In Shopify, open Online Store › Themes, duplicate your theme as a backup, then open the menu next to it and Edit code.
3. In layout/theme.liquid, paste the line just before `</head>`, with the consent lines into Sumly it if your store shows Shopify's cookie banner. Save, and open your store: the visit shows on Visitors.

theme.liquid: Just before </head>.
```
<script defer src="https://sumly.dev/s.js" data-site="sl_your_key"></script>
```

Consent: With Shopify's cookie banner (Settings › Customer privacy): Sumly sets its cookie once the visitor allows analytics. Shopify asks first only where your privacy settings say so.
```
<script>
  window.Shopify.loadFeatures([{ name: 'consent-tracking-api', version: '0.2' }], function (error) {
    if (error) return;
    var tell = function () { window.sumly && sumly.consent(window.Shopify.customerPrivacy.analyticsProcessingAllowed()); };
    tell();
    document.addEventListener('visitorConsentCollected', tell);
  });
</script>
```

Keep in mind: You can edit a theme's code only once the theme is bought, not during a paid theme's trial. A theme update made by Shopify can drop code changes. After updating, check theme.liquid still has the line.

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 Shopify'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.