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.

ElementX/Sources/Mocks/Generated/GeneratedMocks.swift

@@ -4019,6 +4019,70 @@ class ClientProxyMock: ClientProxyProtocol, @unchecked Sendable {
             return removeUserAvatarReturnValue
         }
     }
+    //MARK: - linkNewDeviceService
+
+    var linkNewDeviceServiceUnderlyingCallsCount = 0
+    var linkNewDeviceServiceCallsCount: Int {
+        get {
+            if Thread.isMainThread {
+                return linkNewDeviceServiceUnderlyingCallsCount
+            } else {
+                var returnValue: Int? = nil
+                DispatchQueue.main.sync {
+                    returnValue = linkNewDeviceServiceUnderlyingCallsCount
+                }
+
+                return returnValue!
+            }
+        }
+        set {
+            if Thread.isMainThread {
+                linkNewDeviceServiceUnderlyingCallsCount = newValue
+            } else {
+                DispatchQueue.main.sync {
+                    linkNewDeviceServiceUnderlyingCallsCount = newValue
+                }
+            }
+        }
+    }
+    var linkNewDeviceServiceCalled: Bool {
+        return linkNewDeviceServiceCallsCount > 0
+    }
+
+    var linkNewDeviceServiceUnderlyingReturnValue: LinkNewDeviceService!
+    var linkNewDeviceServiceReturnValue: LinkNewDeviceService! {
+        get {
+            if Thread.isMainThread {
+                return linkNewDeviceServiceUnderlyingReturnValue
+            } else {
+                var returnValue: LinkNewDeviceService? = nil
+                DispatchQueue.main.sync {
+                    returnValue = linkNewDeviceServiceUnderlyingReturnValue
+                }
+
+                return returnValue!
+            }
+        }
+        set {
+            if Thread.isMainThread {
+                linkNewDeviceServiceUnderlyingReturnValue = newValue
+            } else {
+                DispatchQueue.main.sync {
+                    linkNewDeviceServiceUnderlyingReturnValue = newValue
+                }
+            }
+        }
+    }
+    var linkNewDeviceServiceClosure: (() -> LinkNewDeviceService)?
+
+    func linkNewDeviceService() -> LinkNewDeviceService {
+        linkNewDeviceServiceCallsCount += 1
+        if let linkNewDeviceServiceClosure = linkNewDeviceServiceClosure {
+            return linkNewDeviceServiceClosure()
+        } else {
+            return linkNewDeviceServiceReturnValue
+        }
+    }
     //MARK: - deactivateAccount
 
     var deactivateAccountPasswordEraseDataUnderlyingCallsCount = 0

ElementX/Sources/Mocks/SDK/GrantLoginWithQrCodeHandlerSDKMock.swift

@@ -0,0 +1,33 @@
+//
+// 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
+
+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)))
+            }
+        }
+    }
+}