This pull request introduces a comprehensive backup retention feature for local backups, allowing users to limit the number of backup files kept and to manage backup file naming and retention more flexibly. The changes affect both the backend logic and the user interface, and include robust tests to ensure correct behavior.

Backup retention and file management enhancements:

• Added new settings and UI controls to allow users to specify the maximum number of local backups to retain, with options for predefined limits or a custom value. The oldest backups are deleted when the limit is exceeded.
• Updated backup file naming logic to support timestamped files and to only apply retention logic when separate files per backup (timestamping) is enabled.
• Added new settings keys (MaxLocalBackupCount, MaxLocalBackupCountCustom) and their resolution in the settings engine.

User interface improvements:

• Updated the backup settings page to include new controls for backup retention, including a combobox for selecting retention limits and a textbox for custom values, with dynamic enablement and layout.
• Improved the search index for settings to include new backup retention options and updated the description of timestamped backups.

Testing and reliability:

• Added a comprehensive test suite (LocalBackupManagerTests.cs) to verify backup retention logic, file naming, settings integration, and edge cases such as unrelated files and non-Gregorian calendars.

These changes provide users with greater control over backup file management, improve clarity in the UI, and ensure robust, predictable behavior for backup retention.