[Feature Request] Add support for Valve's Steam and Trade Confirmations

Heya, excellent app! I love the fluent design language!

If I may, I have a feature request. I would love if you added support for Steam and steam trade confirmations; that's the only thing that I still need to use a secondary authenticator for, of which there's only [one working version](https://github.com/Jessecar96/SteamDesktopAuthenticator) which last had an update three years ago but functions (though crashes when confirming trades) and a mostly broken, old program called [WinAuth](https://github.com/winauth/winauth). I would be highly grateful if you'd consider adding it so I can completely switch over to Protecc. I really don't want to install the steam app just to authenticate steam.

Regardless, thanks for providing Protecc; it's been a pleasure using it so far

No files, tests, or entry points are named. Start by reviewing Protecc's existing authenticator support and the requirements for Steam trade confirmations, then clarify the integration scope and confirmation flow before implementation. Done means Steam sign-in and trade confirmations work within Protecc without requiring the Steam app.

Written by the indexing model from the issue text.