Add Sumly to a Flutter app

Set up Flutter 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/flutter; read it if anything below is unclear.

Steps:
1. In Sumly, open Visitors and copy your project's public key (`sl_…`). Add the `http` package to your app.
2. Write a small function that posts a screen to `https://sumly.dev/api/v1/events` with the key as a Bearer token and RevenueCat's app user id (`await Purchases.appUserID`) as `user_id`, as into Sumly.
3. Call it from a `NavigatorObserver` on `MaterialApp.navigatorObservers` (or GoRouter's `observers`), so each route that opens counts. The screens show on the project's Sources list.

sumly.dart: One event per call; the events API also takes up to 1,000 in one list.
```
import 'dart:convert';
import 'dart:io' show Platform;
import 'package:http/http.dart' as http;
import 'package:purchases_flutter/purchases_flutter.dart';

Future<void> sumlyScreen(String screen) async {
  final userId = await Purchases.appUserID;
  await http.post(
    Uri.parse('https://sumly.dev/api/v1/events'),
    headers: {'Authorization': 'Bearer sl_your_key', 'Content-Type': 'application/json'},
    body: jsonEncode({'events': [
      {'type': 'screen', 'screen': screen, 'user_id': userId, 'platform': Platform.isIOS ? 'ios' : 'android'},
    ]}),
  );
}
```

Screens: A route without a name is skipped; name your routes, or pass a name of your own.
```
class SumlyObserver extends NavigatorObserver {
  @override
  void didPush(Route route, Route? previousRoute) {
    final name = route.settings.name;
    if (name != null) sumlyScreen(name);
  }
}

MaterialApp(navigatorObservers: [SumlyObserver()], …)
// or GoRouter(observers: [SumlyObserver()], …)
```

Keep in mind: A track event needs a name: `{'type': 'track', 'name': 'trial_started', 'user_id': …}`. An event older than a week, or dated in the future, is kept at the moment it arrives.

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 Flutter'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.