Which page on your site led to an app install, a sign-up and a payment

Set up Web to app 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/web-to-app; read it if anything below is unclear.

Steps:
1. Just the website: keep Sumly's site script on the pages with your store buttons. It adds the visit to each Google Play link and counts every tap on Google Play, App Store and Mac App Store links (`mt=12` or `macappstore://`). Nothing else to change on the site.
2. App with the Sumly SDK: on its first launch the SDK sends `app_installed` by itself. On Android, add `react-native-play-install-referrer` and pass `installReferrer` to `Sumly.init` (Kotlin: the official `com.android.installreferrer` client). On iOS nothing more: the SDK calls Sumly from the phone.
3. App without the SDK: your server sends `app_installed` once per new user with the secret key: on Android the install referrer the app read, on iOS and the Mac (`platform: mac`) the device's IP and country from the sign-up request.

Website: The line you already have. Store links are found by themselves, also ones added later.
```
<script defer src="https://sumly.dev/s.js" data-site="sl_your_key"></script>\n\n<!-- Your store buttons, as they are. The script adds the visit to the Google Play one and counts taps on both: -->\n<a href="https://play.google.com/store/apps/details?id=com.example.app">Google Play</a>\n<a href="https://apps.apple.com/app/example/id1234567890">App Store</a>
```

React Native: One-time app_installed on the first launch. installReferrer is Android only and optional; it needs a development build, not Expo Go.
```
import AsyncStorage from '@react-native-async-storage/async-storage';
import { PlayInstallReferrer } from 'react-native-play-install-referrer';   // Android, optional
import { Sumly } from './sumly';

Sumly.init({
  siteKey: 'sl_your_key',
  storage: AsyncStorage,
  // Android: Google Play's referrer, read once for the first launch's app_installed
  installReferrer: () => new Promise((ok) =>
    PlayInstallReferrer.getInstallReferrerInfo((info, error) => ok(error ? null : info?.installReferrer ?? null))),
});
Sumly.identify(await Purchases.getAppUserID());
```

Kotlin: The same with Google's own install referrer client.
```
// build.gradle: implementation("com.android.installreferrer:installreferrer:2.2")
Sumly.init(this, "sl_your_key", installReferrer = { done ->
    val client = InstallReferrerClient.newBuilder(this).build()
    client.startConnection(object : InstallReferrerStateListener {
        override fun onInstallReferrerSetupFinished(code: Int) {
            done(if (code == InstallReferrerClient.InstallReferrerResponse.OK) client.installReferrer.installReferrer else null)
            client.endConnection()
        }
        override fun onInstallReferrerServiceDisconnected() {}
    })
})
```

Server: Without the SDK, once per new app user, with the user id RevenueCat knows. Neither the referrer nor the IP is kept.
```
POST https://sumly.dev/api/v1/events
Authorization: Bearer sk_sumly_…

{ "events": [
  { "type": "track", "name": "app_installed", "user_id": "tl-48213", "platform": "android",
    "props": { "install_referrer": "utm_source=reddit&utm_medium=referral&sumly_vid=8f1c…" } },
  { "type": "track", "name": "app_installed", "user_id": "tl-48214", "platform": "ios",
    "country": "US", "ip": "203.0.113.7" }
] }
```

So each payment shows which visit it came from: A payment in the app (RevenueCat) by someone who came from your site counts for the source of their first visit, as a web payment does. On Android the link is exact; on iOS it is an estimate, and the page says so.

Keep in mind: Someone who never visited your site cannot be traced to it. On iOS that holds for every install without a tap on your site. Two people tapping on the same network within the hour (an office, a family) tie nothing on iOS: no guess between them. The iOS estimate needs the install within an hour of the tap.

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 Web to app'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.