Add a stubbed OIDC authority (discovery, jwks, /connect/token,
/connect/userinfo) and a fake IBrowser, then cover the full
authorization-code + PKCE flow in LoginPageViewModelTests.
The new LoginPageViewModel(Settings, Func<IBrowser?>) constructor
plus the BrowserFactoryOverride property keep production wiring
unchanged: the existing parameterless ctor and the platform
projects' Platform.CreateBrowser still drive runtime.
