Handling FIREBASE_MEASUREMENT_ID

The `.env.example` contains `VITE_FIREBASE_MEASUREMENT_ID`, but it is actually unused in the codebase. If I may suggest, we need to either:

1. Use it for analytics properly
```ts
const firebaseConfig = {
...
measurementId: import.meta.env.VITE_FIREBASE_MEASUREMENT_ID,
};

const app = initializeApp(firebaseConfig);
const analytics = getAnalytics(app);
```
2. Remove it from `.env.example`

I'll be happy to push the fix once a decision is made!