Add tests for linking a new device. (#4934)

* 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.

ElementX/Sources/Mocks/SDK/GrantLoginWithQrCodeHandlerSDKMock.swift

@@ -1,34 +0,0 @@
-//
-// Copyright 2025 Element Creations Ltd.
-//
-// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
-// Please see LICENSE files in the repository root for full details.
-//
-
-import Foundation
-import MatrixRustSDK
-import MatrixRustSDKMocks
-
-extension GrantLoginWithQrCodeHandlerSDKMock {
-    struct Configuration {
-        var generateDelay: Duration = .seconds(0)
-        var generatedBase64QRCode = """
-        TUFUUklYAgS0yzZ1QVpQ1jlnoxWX3d5jrWRFfELxjS2gN7pz9y+3PABaaHR0
-        cHM6Ly9zeW5hcHNlLW9pZGMubGFiLmVsZW1lbnQuZGV2L19zeW5hcHNlL2Ns
-        aWVudC9yZW5kZXp2b3VzLzAxSFg5SzAwUTFINktQRDQ3RUc0RzFUM1hHACVo
-        dHRwczovL3N5bmFwc2Utb2lkYy5sYWIuZWxlbWVudC5kZXYv
-        """
-    }
-    
-    convenience init(_ configuration: Configuration) {
-        self.init()
-        
-        generateProgressListenerClosure = { listener in
-            Task {
-                try await Task.sleep(for: configuration.generateDelay)
-                let bytes = Data(base64Encoded: configuration.generatedBase64QRCode) ?? Data()
-                try listener.onUpdate(state: .qrReady(qrCode: .fromBytes(bytes: bytes)))
-            }
-        }
-    }
-}