Add Sumly to an iOS or macOS app

Set up Swift and SwiftUI 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/swift; read it if anything below is unclear.

Steps:
1. In Sumly, open Visitors and copy your project's public key (`sl_…`). Download https://sumly.dev/sdk/Sumly.swift and add it to your app's target in Xcode.
2. As the app starts, call `Sumly.configure(siteKey: "sl_…")`. After `Purchases.configure`, and again after each `logIn` or `logOut`, call `Sumly.identify(Purchases.shared.appUserID)`.
3. Count screens: `.sumlyScreen("Paywall")` on a SwiftUI view counts it each time it appears; in UIKit, `Sumly.screen` in `viewDidAppear`. In your privacy manifest, declare UserDefaults with reason CA92.1, if it is not there already.

Setup: In the App's init, or didFinishLaunching.
```
import SwiftUI
import RevenueCat

@main
struct MyApp: App {
    init() {
        Sumly.configure(siteKey: "sl_your_key")
        Purchases.configure(withAPIKey: "appl_…")
        Sumly.identify(Purchases.shared.appUserID)
    }

    var body: some Scene {
        WindowGroup { ContentView() }
    }
}

// after signing in or out
_ = try await Purchases.shared.logIn(user.id)
Sumly.identify(Purchases.shared.appUserID)
```

Screens: SwiftUI's NavigationStack has no hook for a change of screen, so each screen says so as it appears.
```
// SwiftUI
PaywallView()
    .sumlyScreen("Paywall")

// UIKit
override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    Sumly.screen("Paywall")
}

// events
Sumly.track("trial_started", props: ["plan": "yearly"])
```

Consent: Where your app asks first.
```
Sumly.configure(siteKey: "sl_your_key", waitForConsent: true)

Sumly.consent(true)    // when the person agrees
Sumly.consent(false)   // when they refuse, or take it back
```

Own domain: Optional: your own address takes app events too.
```
Sumly.configure(siteKey: "sl_your_key", endpoint: URL(string: "https://signal.example.com"))
```

Keep in mind: No advertising id and no tracking across apps, so no App Tracking Transparency prompt. In App Privacy, declare Product Interaction, User ID and Coarse Location, used for analytics. The queue is kept in UserDefaults: offline, screens and events wait and go once the network is back.

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 Swift and SwiftUI'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.