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/Controls/ModalShell.axaml.cs

@@ -1,3 +1,4 @@
+using System.Globalization;
 using System.Windows.Input;
 using Avalonia;
 using Avalonia.Controls;
@@ -5,6 +6,7 @@ using Avalonia.Controls.Primitives;
 using Avalonia.Input;
 using Avalonia.Interactivity;
 using Avalonia.Media;
+using ClaudeDo.Ui.Converters;
 
 namespace ClaudeDo.Ui.Views.Controls;
 
@@ -84,9 +86,8 @@ public class ModalShell : ContentControl
     {
         if (_maxIcon is null || _window is null) return;
         var maximized = _window.WindowState == WindowState.Maximized;
-        var key = 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(
+            maximized ? "mdi:WindowRestore" : "mdi:WindowMaximize", typeof(Geometry), null, CultureInfo.InvariantCulture);
         if (_maxButton is not null)
         {
             var tipKey = maximized ? "modals.diff.restore" : "modals.diff.maximize";