frontend: enforce tabs vs spaces in eslint config

[alxndrsn]

Noted in https://github.com/getodk/central-frontend/pull/1728/changes, some eslint config files use tabs, and some spaces. No big deal particularly, but at least for a couple of config files, there's no enforcement of this decision:

• apps/forms/eslint.config.js
• packages/eslint.config.js

[matthew-white]

Everything that's Central only (not Web Forms or shared with Web Forms) should use spaces in .js files, not tabs. For anything that Web Forms uses, I'm very happy for the Web Forms team to use whatever white space they prefer.

I agree that we should lint all *.config.js files.

I'm going to go ahead and move this issue to the backlog, but definitely feel free to pull it into the current release cycle at any point.