refactor(ui): migrate PlanningDiffView to ModalShell

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/Planning/PlanningDiffView.axaml

@@ -1,6 +1,7 @@
 <Window xmlns="https://github.com/avaloniaui"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:vm="using:ClaudeDo.Ui.ViewModels.Planning"
+        xmlns:ctl="using:ClaudeDo.Ui.Views.Controls"
         x:Class="ClaudeDo.Ui.Views.Planning.PlanningDiffView"
         x:DataType="vm:PlanningDiffViewModel"
         Title="Planning — Combined diff"
@@ -14,32 +15,11 @@
     <KeyBinding Gesture="Escape" Command="{Binding CloseCommand}"/>
   </Window.KeyBindings>
 
-  <Border Background="{DynamicResource SurfaceBrush}"
-          BorderBrush="{DynamicResource LineBrush}"
-          BorderThickness="1">
-    <Grid RowDefinitions="36,Auto,*">
+  <ctl:ModalShell Title="PLANNING — COMBINED DIFF" CloseCommand="{Binding CloseCommand}">
 
-      <!-- Title bar / drag handle -->
-      <Border Grid.Row="0"
-              x:Name="TitleBar"
-              Background="{DynamicResource Surface2Brush}"
-              BorderBrush="{DynamicResource LineBrush}"
-              BorderThickness="0,0,0,1"
-              PointerPressed="TitleBar_PointerPressed">
-        <Grid ColumnDefinitions="*,Auto" Margin="14,0">
-          <TextBlock Classes="title" Text="Planning — Combined diff"
-                     VerticalAlignment="Center"/>
-          <Button Grid.Column="1"
-                  Classes="icon-btn"
-                  Content="✕"
-                  FontSize="{StaticResource FontSizeBody}"
-                  Command="{Binding CloseCommand}"
-                  VerticalAlignment="Center"/>
-        </Grid>
-      </Border>
-
-      <!-- Toolbar row -->
-      <StackPanel Grid.Row="1"
+    <!-- Toolbar row -->
+    <DockPanel>
+      <StackPanel DockPanel.Dock="Top"
                   Orientation="Horizontal"
                   Spacing="8"
                   Margin="8,6">
@@ -54,7 +34,7 @@
       </StackPanel>
 
       <!-- Two-pane body -->
-      <Grid Grid.Row="2" ColumnDefinitions="240,*">
+      <Grid ColumnDefinitions="240,*">
 
         <!-- Subtask list (left pane) -->
         <Border Grid.Column="0"
@@ -95,6 +75,7 @@
         </Grid>
 
       </Grid>
-    </Grid>
-  </Border>
+    </DockPanel>
+
+  </ctl:ModalShell>
 </Window>

src/ClaudeDo.Ui/Views/Planning/PlanningDiffView.axaml.cs

@@ -1,6 +1,4 @@
 using Avalonia.Controls;
-using Avalonia.Input;
-using CommunityToolkit.Mvvm.Input;
 using ClaudeDo.Ui.ViewModels.Planning;
 
 namespace ClaudeDo.Ui.Views.Planning;
@@ -18,10 +16,4 @@ public partial class PlanningDiffView : Window
         if (DataContext is PlanningDiffViewModel vm)
             vm.CloseAction = Close;
     }
-
-    private void TitleBar_PointerPressed(object? sender, PointerPressedEventArgs e)
-    {
-        if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
-            BeginMoveDrag(e);
-    }
 }