Fix autosync toggle disable functionality

- Fix service instance management to use global instance for stopping autosync
- Add automatic saving when toggle is changed (no manual save required)
- Fix validation issue where custom sync type without cron expression caused 400 error
- Add comprehensive debugging and error handling
- Ensure .env file is properly updated with AUTO_SYNC_ENABLED value
- Improve service lifecycle management with proper state cleanup
- Add fallback logic for invalid sync interval configurations

Resolves issue where disabling autosync in GUI didn't update .env file or stop service

.env.example

@@ -27,3 +27,12 @@ AUTH_ENABLED=false
 AUTH_SETUP_COMPLETED=false
 JWT_SECRET=
 DATABASE_URL="file:/opt/ProxmoxVE-Local/data/settings.db"
+AUTO_SYNC_ENABLED=false
+SYNC_INTERVAL_TYPE=
+SYNC_INTERVAL_PREDEFINED=
+AUTO_DOWNLOAD_NEW=
+AUTO_UPDATE_EXISTING=
+NOTIFICATION_ENABLED=
+APPRISE_URLS=
+LAST_AUTO_SYNC=
+SYNC_INTERVAL_CRON=