MainWP Updraft Plus扩展 5.1.1

* Fixed: An issue where reporting email settings are not saved or pushed to child sites.
* Added: The "all-sites" view to Existing Backups page.
* Updated: Text domain references to 'mainwp-updraftplus-extension' across multiple files.

* Fixed: Resolved an issue that could prevent backup files from downloading properly.
* Fixed: Corrected a PHP notice related to _load_textdomain_just_in_time being called incorrectly.
* Fixed: Fixed a UI issue where optional toggles for "Include in files backup" only appeared after saving settings.
* Fixed: Included the missing Must-Use Plugins backup option.
* Fixed: Resolved a jQuery error that could occur when saving settings.
* Added: Added the ajax_get_site_backups_html AJAX endpoint to load a site's backups dynamically from the dropdown with loading and error handling.
* Added: Introduced placeholder element for more consistent empty states across the interface.
* Updated: Refined backup management UI elements, including button styles, padded segments, modal close icon, and schedule and plugin table markup for a cleaner experience.

* Updated: MainWP 4.3.1 compatibility

* Updated: Amazon S3 key input type
* Preventative: Multiple security improvements