Cleanup following the AppMediator introduction (#2723)

- stop using multiple background task, the appCoordinator sync one is enough for the whole app
- move the AppMeditor to the MainActor
- expose the WindowManager through the AppMediator
- hide sensitive WindowManager API behind a different protocol
- remove the now unnecessary `BackgroundTaskService`

NSE/Sources/Other/NSEUserSession.swift

@@ -22,8 +22,7 @@ final class NSEUserSession {
     private let notificationClient: NotificationClient
     private let userID: String
     private(set) lazy var mediaProvider: MediaProviderProtocol = MediaProvider(mediaLoader: MediaLoader(client: baseClient),
-                                                                               imageCache: .onlyOnDisk,
-                                                                               backgroundTaskService: nil)
+                                                                               imageCache: .onlyOnDisk)
     private let delegateHandle: TaskHandle?
 
     init(credentials: KeychainCredentials, clientSessionDelegate: ClientSessionDelegate) async throws {

NSE/SupportingFiles/target.yml

@@ -97,8 +97,6 @@ targets:
     - path: ../../ElementX/Sources/Other/TestablePreview.swift
     - path: ../../ElementX/Sources/Other/Pills/PlainMentionBuilder.swift
     - path: ../../ElementX/Sources/Other/Pills/PillConstants.swift
-    - path: ../../ElementX/Sources/Services/BackgroundTasks/BackgroundTaskProtocol.swift
-    - path: ../../ElementX/Sources/Services/BackgroundTasks/BackgroundTaskServiceProtocol.swift
     - path: ../../ElementX/Sources/Services/Keychain/KeychainController.swift
     - path: ../../ElementX/Sources/Services/Keychain/KeychainControllerProtocol.swift
     - path: ../../ElementX/Sources/Services/Media/Provider