Replaces the group tab and server list RecyclerView in MainActivity with a ViewPager2-based implementation using fragments. Adds GroupPagerAdapter and GroupServerFragment for per-group server lists, introduces BaseFragment for fragment view binding, and updates related logic in MainViewModel and SettingsChangeManager. Updates dependencies to include ViewPager2 and Fragment KTX, and modifies layouts to support the new structure.
Introduces a new BackupActivity for configuration backup and restore, supporting both local storage and WebDAV servers. Adds WebDavManager and WebDavConfig for handling WebDAV operations, updates the navigation drawer and menu, and refactors AboutActivity to remove backup/restore logic. Includes new layouts and string resources for backup options and WebDAV settings.
Raised minSdk to 24 and removed conditional code for pre-Nougat Android versions. Cleaned up context wrapping, notification, and service start logic to use modern APIs. Also removed unnecessary @RequiresApi annotations and legacy feature declarations from the manifest.
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