feat: read baseUrl from an environment variable

[andylolz]

Fixes #40.

You can use this by starting the development server with:

REACT_APP_BASE_URL=http://localhost:3000/api npm run start

…or with a .env file.

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Pull request checklist

• [x] I have linked my PR to an issue
• [x] I’ve used conventional commits
• [x] My branch is up-to-date with main
• [ ] Where appropriate, I have added or updated tests
• [ ] Where appropriate, I have updated documentation to reflect my changes

[andylolz]

Adding Simon here too, because I don’t know if this is the preferred way to do this in React.

I got this approach from here: https://create-react-app.dev/docs/adding-custom-environment-variables/