Add Node 18 LTS to Flake for example frontends (#355)

# Description

Websocket example app depends on Nodejs, and (from a side conversation)
a minimum version. This PR adds Node 18 LTS to to the Nix Flake, and
rebuilds the Websocket relay lockfile. This means that if you run `nix
develop`, you will have a consistent version of Node on your `PATH`.

## Type of change

- [x] New feature (non-breaking change that adds functionality)

## Test plan (required)

`#worksonmymachine`?

## Screenshots/Screencaps

<img width="1152" alt="Screenshot 2023-10-04 at 23 11 57"
src="https://github.com/ipvm-wg/homestar/assets/1052016/c577c47a-c207-44ad-bd08-599dfb025ba1">