Preface: You don't have to believe me, even though you can look it up and verify it in the browser and our source code. But you should consider one thing: If you don't trust us in regards to Steam, then you shouldn't install any software from us anyway, which makes the whole question pointless!
The login to Steam is used in two places: Steam linking and password reset via Steam.
In both cases the following happens (and you can verify this in your browser yourself):
The whole process uses a public internet standard called OpenID 2.0. This is also described in the Steam API documentation.
The whole OpenID 2.0 process and login just verifies who you are on Steam. At no moment in time do we get control over your account!
tl;dr we ask Steam if you have the game, Steam asks you for credentials, then Steam tells us what games you have and what's your ID. We never receive your Steam credentials.