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.

Examples/AdvancedExample/Models/AdvancedConfiguration.cs

@@ -223,7 +223,7 @@ public class NetworkSettings : INotifyPropertyChanged
 {
     private string _hostname = "api.example.com";
     private int _port = 443;
-    private bool _useProxy = false;
+    private bool _useProxy;
     private string _proxyAddress = string.Empty;
 
     [Display(Name = "Hostname", Description = "Server hostname or IP address")]

Examples/AdvancedExample/Program.cs

@@ -3,7 +3,7 @@ using System;
 
 namespace AdvancedExample;
 
-class Program
+sealed class Program
 {
     [STAThread]
     public static void Main(string[] args) => BuildAvaloniaApp()

Examples/QuickStartExample/Program.cs

@@ -3,7 +3,7 @@ using Avalonia;
 
 namespace QuickStartExample;
 
-class Program
+sealed class Program
 {
     [STAThread]
     public static void Main(string[] args) => BuildAvaloniaApp()