Shopify revenue and refunds, traced to the visit that bought

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/integrations/shopify; read it if anything below is unclear.

Steps:
1. In the Shopify Dev Dashboard (dev.shopify.com, signed in as the store's owner): Create app, name it Sumly. In its version, give it the Admin API scopes read_orders and read_all_orders only, release it and install it on your store.
2. Paste the app's Client ID and Client secret from its Settings, with the store's myshopify.com address. Sumly asks Shopify for a token that lasts a day, and asks again when it runs out.
3. An older custom app made in the store admin before 2026 works too: give it the same two scopes and paste its shpat_ token instead, leaving the Client ID empty.
4. To trace orders to visits, add the theme.liquid snippet shown with Sumly's Shopify integration: it puts the visitor on the cart as the attribute `sumly_visitor_id`.

theme.liquid: Puts the Sumly visitor on the cart, so the order counts for the visit that brought it.
```
<script>
  // Sumly: remember which visit a Shopify order came from.
  addEventListener('load', function () {
    var id = window.sumly && window.sumly.visitorId;
    if (!id || sessionStorage.getItem('sumly_cart') === id) return;
    fetch(window.Shopify && Shopify.routes ? Shopify.routes.root + 'cart/update.js' : '/cart/update.js', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({ attributes: { sumly_visitor_id: id } })
    }).then(function () { sessionStorage.setItem('sumly_cart', id); });
  });
</script>
```

So each payment shows which visit it came from: Add the snippet below to your theme (Online Store → Themes → Edit code → theme.liquid, before </body>), on a store that has the Sumly script in its head. It puts the visitor on the cart as the attribute sumly_visitor_id; Shopify copies cart attributes onto the order, and Sumly reads them back. In the EU and UK there is a visitor only after they agree.

Keep in mind: Built from Shopify's GraphQL Admin API documentation and tested against responses shaped like it; it has not yet been tried on a live store. Tell us if a number looks off. No MRR: Shopify lets an app read only the subscription contracts it made itself, so subscriptions from Shopify Subscriptions, Recharge or other apps cannot be read. Their orders count as revenue, marked as subscription orders. Fees show only for Shopify Payments; other gateways report none to Shopify. Without read_all_orders, Shopify shows an app only the last 60 days of orders.

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.
- The Shopify key goes into Sumly (Projects and payments), not into this project: Sumly reads Shopify itself. Your part in the code is only carrying the visitor into checkout, as above.
- 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.