style(ui): Material Design icons for window chrome, modal shell and Mission Control

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/MainWindow.axaml.cs

@@ -1,3 +1,4 @@
+using System.Globalization;
 using System.Linq;
 using System.Threading.Tasks;
 using Avalonia;
@@ -5,6 +6,7 @@ using Avalonia.Controls;
 using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.Media;
+using ClaudeDo.Ui.Converters;
 using ClaudeDo.Ui.Localization;
 using ClaudeDo.Ui.Services;
 using ClaudeDo.Ui.ViewModels;
@@ -64,9 +66,8 @@ public partial class MainWindow : Window
 
     private void UpdateMaxIcon()
     {
-        var key = WindowState == WindowState.Maximized ? "Icon.WinRestore" : "Icon.WinMax";
-        if (this.TryFindResource(key, out var geometry) && geometry is Geometry g)
-            MaxIcon.Data = g;
+        MaxIcon.Data = (Geometry?)IconKeyConverter.Instance.Convert(
+            WindowState == WindowState.Maximized ? "mdi:WindowRestore" : "mdi:WindowMaximize", typeof(Geometry), null, CultureInfo.InvariantCulture);
     }
 
     private void OnDataContextChanged(object? sender, EventArgs e)