Refactor PropertyGrid UI, converters, and solution setup

- Improved PropertyGrid.axaml formatting and control styles; ensured spinner buttons are always visible and ColorPicker layouts are flexible; disabled horizontal scrolling in property list.
- Reformatted property editors and group/category display for clarity.
- Made all value converters sealed, added XML docs, and updated ConvertBack to throw NotImplementedException where unsupported.
- Enhanced search logic to use StringComparison.OrdinalIgnoreCase.
- Minor code style fixes (e.g., char literals for string checks).
- Removed default value assignment for _useProxy in AdvancedConfiguration.cs.
- Updated Program.cs in examples to use sealed Program classes.
- Added PropertyGrid.slnx to organize solution and projects.

Avalonia.PropertyGrid/Editors/PropertyEditors.cs

@@ -266,7 +266,7 @@ public class DateTimeEditor : PropertyEditor
 /// <summary>
 /// Converter for DateTime to DateTimeOffset
 /// </summary>
-internal class DateTimeToDateTimeOffsetConverter : Avalonia.Data.Converters.IValueConverter
+internal sealed class DateTimeToDateTimeOffsetConverter : Avalonia.Data.Converters.IValueConverter
 {
     public object? Convert(object? value, Type targetType, object? parameter, System.Globalization.CultureInfo culture)
     {