Refactor: Remove background task management and related hooks

- Deleted `useBackgroundTasks.ts` hook and its associated logic for managing background tasks.
- Removed `backgroundTaskManager.ts` service and all related task definitions and registrations.
- Cleaned up `Podfile.lock` and `package.json` to remove unused dependencies related to background tasks.
- Updated iOS project files to eliminate references to removed background task components.
- Added new background fetch identifier in `Info.plist` for future use.

ios/digitalpilates/Info.plist

@@ -5,6 +5,7 @@
 	<key>BGTaskSchedulerPermittedIdentifiers</key>
 	<array>
 		<string>com.expo.modules.backgroundtask.processing</string>
+		<string>com.anonymous.digitalpilates.backgroundfetch</string>
 	</array>
 	<key>CADisableMinimumFrameDurationOnPhone</key>
 	<true/>