* Replace GrantLoginWithQrCodeHandlerSDKMock with LinkNewDeviceServiceMock.
Add tests for all initial states on the QRCodeLoginScreen.
* Add tests for linking both mobile and desktop devices.
* Add UI tests for linking a new device.
* Don't show the Link Desktop Computer button when running on macOS.
This mirrors the decision to hide the Sign In With QR Code button on the start screen.
