Update heartbeat right away instead of waiting 20 seconds

[cinnamon-bun]

https://github.com/earthstar-project/earthstar-status/blob/main/src/App.tsx#L131-L154

The heartbeat writer is in a setInterval which runs every 20 seconds, but it should also run right away when the page is first loaded.

Pull the contents of the setInterval out into a smaller function called writeHeartbeat, and run that function in the setInterval as well as just before it, as the first line inside the useEffect