iOS: native upload and download services (background URLSession)

Swift implementation of the upload_service/download_service channels:
background PUT/GET against the account's WebDAV root, one summary local
notification per batch, Downloads visible in the Files app. Pure helpers
covered by RunnerTests.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

ios/Runner/AppDelegate.swift

@@ -7,10 +7,25 @@ import UIKit
     _ application: UIApplication,
     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
   ) -> Bool {
+    TransferManager.shared.reconnect()
     return super.application(application, didFinishLaunchingWithOptions: launchOptions)
   }
 
+  override func application(
+    _ application: UIApplication,
+    handleEventsForBackgroundURLSession identifier: String,
+    completionHandler: @escaping () -> Void
+  ) {
+    if identifier == TransferManager.sessionIdentifier {
+      TransferManager.shared.backgroundCompletionHandler = completionHandler
+    } else {
+      super.application(
+        application, handleEventsForBackgroundURLSession: identifier, completionHandler: completionHandler)
+    }
+  }
+
   func didInitializeImplicitFlutterEngine(_ engineBridge: FlutterImplicitEngineBridge) {
     GeneratedPluginRegistrant.register(with: engineBridge.pluginRegistry)
+    NativeServices.register(messenger: engineBridge.applicationRegistrar.messenger())
   }
 }