feat(ui): add reusable ModalShell control

src/ClaudeDo.Ui/Views/Controls/ModalShell.axaml

@@ -0,0 +1,43 @@
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                    xmlns:ctl="using:ClaudeDo.Ui.Views.Controls">
+  <ControlTheme x:Key="{x:Type ctl:ModalShell}" TargetType="ctl:ModalShell">
+    <Setter Property="Template">
+      <ControlTemplate>
+        <Border Background="{DynamicResource SurfaceBrush}"
+                BorderBrush="{DynamicResource LineBrush}"
+                BorderThickness="1"
+                CornerRadius="{DynamicResource ModalCornerRadius}"
+                ClipToBounds="True">
+          <DockPanel>
+            <Border Name="PART_TitleBar" DockPanel.Dock="Top" Height="36"
+                    Background="{DynamicResource DeepBrush}"
+                    BorderBrush="{DynamicResource LineBrush}"
+                    BorderThickness="0,0,0,1">
+              <Grid ColumnDefinitions="*,Auto" Margin="14,0">
+                <TextBlock Text="{TemplateBinding Title}"
+                           FontFamily="{DynamicResource MonoFont}"
+                           FontSize="{DynamicResource FontSizeMono}"
+                           LetterSpacing="1.4"
+                           Foreground="{DynamicResource TextBrush}"
+                           VerticalAlignment="Center"/>
+                <Button Grid.Column="1" Classes="icon-btn" Content="✕"
+                        FontSize="{DynamicResource FontSizeBody}"
+                        Command="{TemplateBinding CloseCommand}"
+                        VerticalAlignment="Center"/>
+              </Grid>
+            </Border>
+            <Border Name="PART_Footer" DockPanel.Dock="Bottom"
+                    Background="{DynamicResource DeepBrush}"
+                    BorderBrush="{DynamicResource LineBrush}"
+                    BorderThickness="0,1,0,0"
+                    IsVisible="{TemplateBinding Footer, Converter={x:Static ObjectConverters.IsNotNull}}">
+              <ContentPresenter Content="{TemplateBinding Footer}" Margin="16,8"/>
+            </Border>
+            <ContentPresenter Content="{TemplateBinding Content}"/>
+          </DockPanel>
+        </Border>
+      </ControlTemplate>
+    </Setter>
+  </ControlTheme>
+</ResourceDictionary>

src/ClaudeDo.Ui/Views/Controls/ModalShell.axaml.cs

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+using System;
+using System.Windows.Input;
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Primitives;
+using Avalonia.Input;
+
+namespace ClaudeDo.Ui.Views.Controls;
+
+/// <summary>Reusable modal chrome: titlebar (drag + close) wrapping a body and optional footer.</summary>
+public class ModalShell : ContentControl
+{
+    public static readonly StyledProperty<string?> TitleProperty =
+        AvaloniaProperty.Register<ModalShell, string?>(nameof(Title));
+
+    public static readonly StyledProperty<object?> FooterProperty =
+        AvaloniaProperty.Register<ModalShell, object?>(nameof(Footer));
+
+    public static readonly StyledProperty<ICommand?> CloseCommandProperty =
+        AvaloniaProperty.Register<ModalShell, ICommand?>(nameof(CloseCommand));
+
+    public string? Title { get => GetValue(TitleProperty); set => SetValue(TitleProperty, value); }
+    public object? Footer { get => GetValue(FooterProperty); set => SetValue(FooterProperty, value); }
+    public ICommand? CloseCommand { get => GetValue(CloseCommandProperty); set => SetValue(CloseCommandProperty, value); }
+
+    protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
+    {
+        base.OnApplyTemplate(e);
+        if (e.NameScope.Find<Border>("PART_TitleBar") is { } bar)
+            bar.PointerPressed += OnTitleBarPressed;
+    }
+
+    private void OnTitleBarPressed(object? sender, PointerPressedEventArgs e)
+    {
+        if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed && VisualRoot is Window w)
+            w.BeginMoveDrag(e);
+    }
+}