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/Controls/PropertyGrid.axaml.cs

@@ -288,11 +288,10 @@ public class PropertyGrid : TemplatedControl
         // Apply search filter
         if (!string.IsNullOrWhiteSpace(_searchText))
         {
-            var searchLower = _searchText.ToLowerInvariant();
             filtered = filtered.Where(p =>
-                p.DisplayName.ToLowerInvariant().Contains(searchLower) ||
-                (p.Category?.ToLowerInvariant().Contains(searchLower) ?? false) ||
-                (p.Description?.ToLowerInvariant().Contains(searchLower) ?? false));
+                p.DisplayName.Contains(_searchText, StringComparison.OrdinalIgnoreCase) ||
+                (p.Category?.Contains(_searchText, StringComparison.OrdinalIgnoreCase) ?? false) ||
+                (p.Description?.Contains(_searchText, StringComparison.OrdinalIgnoreCase) ?? false));
         }
 
         // Populate flat list