Add a service and flow coordinator for the LinkNewDevice feature. (#4859)

* Add a LinkNewDeviceService that exposes the SDK's grant QR code login methods.

* Add a flow coordinator for linking a new device.

Changes the presentation too.

UITests/Sources/LinkNewDeviceTests.swift

@@ -0,0 +1,26 @@
+//
+// 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 XCTest
+
+@MainActor
+class LinkNewDeviceTests: XCTestCase {
+    enum Step {
+        static let selectDevice = 1
+        static let dismissed = 99
+    }
+    
+    func testFlow() async throws {
+        let app = Application.launch(.linkNewDevice)
+        try await app.assertScreenshot(step: Step.selectDevice)
+        
+        let cancelButton = app.buttons[A11yIdentifiers.linkNewDeviceScreen.cancel]
+        cancelButton.tap()
+        
+        try await app.assertScreenshot(step: Step.dismissed)
+    }
+}