Add Sumly to a Capacitor or Ionic app

Set up Capacitor and Ionic 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/capacitor; read it if anything below is unclear.

Steps:
1. In Sumly, open Visitors and copy your project's public key (`sl_…`).
2. Add the small function into Sumly: it 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.getAppUserID()).appUserID`) as `user_id`. Sumly's API answers apps on any origin, so plain fetch works.
3. Call it on each change of page: Angular's `NavigationEnd`, Vue Router's `afterEach` or React Router's `useLocation`. The screens show on the project's Sources list.

sumly.ts: The platform comes from Capacitor; on the web, use the website script instead.
```
import { Capacitor } from '@capacitor/core';
import { Purchases } from '@revenuecat/purchases-capacitor';

export async function sumlyScreen(screen: string) {
  if (!Capacitor.isNativePlatform()) return;
  const { appUserID } = await Purchases.getAppUserID();
  await fetch('https://sumly.dev/api/v1/events', {
    method: 'POST',
    headers: { Authorization: 'Bearer sl_your_key', 'Content-Type': 'application/json' },
    body: JSON.stringify({ events: [{ type: 'screen', screen, user_id: appUserID, platform: Capacitor.getPlatform() }] }),
  });
}
```

Routers: Whichever your app uses.
```
// Angular
this.router.events.subscribe((e) => { if (e instanceof NavigationEnd) sumlyScreen(e.urlAfterRedirects); });

// Vue Router
router.afterEach((to) => sumlyScreen(to.path));

// React Router
const location = useLocation();
useEffect(() => { sumlyScreen(location.pathname); }, [location]);
```

Keep in mind: Do not load the website script inside the app: its pages would count as a website on localhost, not as an app. With CapacitorHttp turned on in capacitor.config, the same fetch goes through the native network layer; nothing else changes.

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 Capacitor and Ionic'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.