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/Models/PropertyItem.cs

@@ -182,7 +182,7 @@ public class PropertyItem : INotifyPropertyChanged
                 try
                 {
                     var hexString = value?.Trim() ?? "";
-                    if (!hexString.StartsWith("#"))
+                    if (!hexString.StartsWith('#'))
                         hexString = "#" + hexString;
 
                     Value = Media.Color.Parse(hexString);