Add functionality to manage custom holiday dates in the settings window, including:

- New custom dates section with scrollable list view
- Ability to add single dates via calendar picker
- Support for adding date ranges with start/end date selection
- Option to remove individual custom dates
- Clear all custom dates functionality
- Chronological sorting of custom dates
- Improved UI layout with separators and bold section headers

Additional UI improvements:
- Increased settings window height to accommodate new content
- Added minimum scroll container height for better visibility
- Improved date formatting for better readability

Technical changes:
- Added new imports for time, sort, and layout packages
- Implemented custom date management functions
- Added modal windows for date selection instead of dialogs
- Enhanced error handling for date range validation