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/Converters/ColorToBrushConverter.cs

@@ -8,8 +8,16 @@ namespace Avalonia.PropertyGrid.Controls;
 /// <summary>
 /// Converts an Avalonia.Media.Color to a SolidColorBrush
 /// </summary>
-public class ColorToBrushConverter : IValueConverter
+public sealed class ColorToBrushConverter : IValueConverter
 {
+    /// <summary>
+    /// Converts a Color to a SolidColorBrush.
+    /// </summary>
+    /// <param name="value">The Color value to convert.</param>
+    /// <param name="targetType">The target type.</param>
+    /// <param name="parameter">Optional parameter.</param>
+    /// <param name="culture">Culture information.</param>
+    /// <returns>A SolidColorBrush with the specified color.</returns>
     public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
     {
         if (value is Color color)
@@ -21,9 +29,17 @@ public class ColorToBrushConverter : IValueConverter
         return new SolidColorBrush(Colors.Transparent);
     }
 
+    /// <summary>
+    /// Converts a SolidColorBrush back to a Color.
+    /// </summary>
+    /// <param name="value">The SolidColorBrush value to convert back.</param>
+    /// <param name="targetType">The target type.</param>
+    /// <param name="parameter">Optional parameter.</param>
+    /// <param name="culture">Culture information.</param>
+    /// <returns>The Color from the brush.</returns>
     public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
     {
-        if (value is SolidColorBrush brush && brush.Color != null)
+        if (value is SolidColorBrush brush)
         {
             return brush.Color;
         }