fix: local sync task never runs on iOS (#21491)

* fix: local sync task never runs on iOS

* chore: rename ios register method

* refactor from using dart callback to dart entrypoint + more logs

* check if file exists before hashing

* reschedule local sync task

* chore: rename background worker logger

* refactor: move file exists check inside repo

---------

Co-authored-by: shenlong-tanwen <139912620+shalong-tanwen@users.noreply.github.com>

mobile/ios/Runner/Background/BackgroundWorker.swift

@@ -86,28 +86,10 @@ class BackgroundWorker: BackgroundWorkerBgHostApi {
    * starts the engine, and sets up a timeout timer if specified.
    */
   func run() {
-    // Retrieve the callback handle stored by the main Flutter app
-    // This handle points to the Flutter function that should be executed in the background
-    let callbackHandle = Int64(UserDefaults.standard.string(
-      forKey: BackgroundWorkerApiImpl.backgroundUploadCallbackHandleKey) ?? "0") ?? 0
-
-    if callbackHandle == 0 {
-      // Without a valid callback handle, we cannot start the Flutter background execution
-      complete(success: false)
-      return
-    }
-    
-    // Use the callback handle to retrieve the actual Flutter callback information
-    guard let callback = FlutterCallbackCache.lookupCallbackInformation(callbackHandle) else {
-      // The callback handle is invalid or the callback was not found
-      complete(success: false)
-      return
-    }
-           
     // Start the Flutter engine with the specified callback as the entry point
     let isRunning = engine.run(
-      withEntrypoint: callback.callbackName,
-      libraryURI: callback.callbackLibraryPath
+      withEntrypoint: "backgroundSyncNativeEntrypoint",
+      libraryURI: "package:immich_mobile/domain/services/background_worker.service.dart"
     )
     
     // Verify that the Flutter engine started successfully
@@ -127,7 +109,7 @@ class BackgroundWorker: BackgroundWorkerBgHostApi {
     if maxSeconds != nil {
         // Schedule a timer to cancel the task after the specified timeout period
         Timer.scheduledTimer(withTimeInterval: TimeInterval(maxSeconds!), repeats: false) { _ in
-          self.cancel()
+          self.close()
         }
     }
   }
@@ -156,7 +138,7 @@ class BackgroundWorker: BackgroundWorkerBgHostApi {
    * Sends a cancel signal to the Flutter side and sets up a fallback timer to ensure
    * the completion handler is eventually called even if Flutter doesn't respond.
    */
-  func cancel() {
+  func close() {
     if isComplete {
       return
     }
@@ -182,7 +164,7 @@ class BackgroundWorker: BackgroundWorkerBgHostApi {
   private func handleHostResult(result: Result<Void, PigeonError>) {
     switch result {
       case .success(): self.complete(success: true)
-      case .failure(_): self.cancel()
+      case .failure(_): self.close()
     }
   }