[auth] refactor: auth to use default browser

[M4rcoToni]

Why

I was trying to login but the BrowserWindow window function of electron use always Safari, but my default browser is Arc.
Maybe be this is a common issue with mac users.

How

I replaced the browser approach with Linking.openURL()... still working on it

Test Plan

1. build and run the app
2. open Settings and trigger the login flow
3. verify the auth URL opens in your default browser (not Safari / an embedded window)
4. complete sign-in in the browser
5. verify the app receives the callback and completes login successfully