fix(ui): make overview modal resizable; add diff content pane

Drop outer Border wrapper in WorktreesOverviewModalView so Avalonia edge
resize handles reach the window frame. Add split pane to WorktreeModalView
with file tree on left and per-file unified diff on right; wire SelectedNode
via SelectedItem TwoWay binding + SelectionChanged fallback; add
GetFileDiffAsync to GitService.

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

src/ClaudeDo.Ui/Views/Modals/WorktreeModalView.axaml

@@ -4,12 +4,13 @@
         x:Class="ClaudeDo.Ui.Views.Modals.WorktreeModalView"
         x:DataType="vm:WorktreeModalViewModel"
         Title="Worktree"
-        Width="640" Height="720"
+        Width="1100" Height="720"
+        MinWidth="640" MinHeight="400"
         WindowStartupLocation="CenterOwner"
         SystemDecorations="None"
         ExtendClientAreaToDecorationsHint="True"
         Background="Transparent"
-        CanResize="False"
+        CanResize="True"
         TransparencyLevelHint="AcrylicBlur">
 
   <Window.KeyBindings>
@@ -39,27 +40,51 @@
                    TextTrimming="CharacterEllipsis"/>
       </Border>
 
-      <!-- File tree -->
-      <TreeView DockPanel.Dock="Top" ItemsSource="{Binding Root}"
-                Background="Transparent" Margin="8,0,8,8">
-        <TreeView.ItemTemplate>
-          <TreeDataTemplate DataType="vm:WorktreeNodeViewModel"
-                            ItemsSource="{Binding Children}">
-            <StackPanel Orientation="Horizontal" Spacing="8">
-              <TextBlock Text="{Binding Name}"
-                         FontFamily="{DynamicResource MonoFont}" FontSize="12"
-                         Foreground="{DynamicResource TextBrush}"/>
-              <Border Tag="{Binding Status}" CornerRadius="3" Padding="4,0"
-                      VerticalAlignment="Center"
-                      IsVisible="{Binding Status, Converter={x:Static ObjectConverters.IsNotNull}}">
-                <TextBlock Text="{Binding Status}"
-                           FontFamily="{DynamicResource MonoFont}" FontSize="10"
+      <!-- Split: file tree | splitter | diff pane -->
+      <Grid ColumnDefinitions="260,4,*">
+
+        <!-- Left: file tree -->
+        <TreeView x:Name="FileTree"
+                  Grid.Column="0"
+                  ItemsSource="{Binding Root}"
+                  SelectedItem="{Binding SelectedNode, Mode=TwoWay}"
+                  Background="Transparent"
+                  Margin="8,0,4,8">
+          <TreeView.ItemTemplate>
+            <TreeDataTemplate DataType="vm:WorktreeNodeViewModel"
+                              ItemsSource="{Binding Children}">
+              <StackPanel Orientation="Horizontal" Spacing="8">
+                <TextBlock Text="{Binding Name}"
+                           FontFamily="{DynamicResource MonoFont}" FontSize="12"
                            Foreground="{DynamicResource TextBrush}"/>
-              </Border>
-            </StackPanel>
-          </TreeDataTemplate>
-        </TreeView.ItemTemplate>
-      </TreeView>
+                <Border Tag="{Binding Status}" CornerRadius="3" Padding="4,0"
+                        VerticalAlignment="Center"
+                        IsVisible="{Binding Status, Converter={x:Static ObjectConverters.IsNotNull}}">
+                  <TextBlock Text="{Binding Status}"
+                             FontFamily="{DynamicResource MonoFont}" FontSize="10"
+                             Foreground="{DynamicResource TextBrush}"/>
+                </Border>
+              </StackPanel>
+            </TreeDataTemplate>
+          </TreeView.ItemTemplate>
+        </TreeView>
+
+        <!-- Splitter -->
+        <GridSplitter Grid.Column="1" ResizeDirection="Columns" Background="{DynamicResource LineBrush}"/>
+
+        <!-- Right: diff content -->
+        <ScrollViewer Grid.Column="2" Padding="8"
+                      HorizontalScrollBarVisibility="Auto"
+                      VerticalScrollBarVisibility="Auto"
+                      Margin="4,0,8,8">
+          <SelectableTextBlock Text="{Binding SelectedFileDiff}"
+                               FontFamily="{DynamicResource MonoFont}"
+                               FontSize="11"
+                               Foreground="{DynamicResource TextBrush}"
+                               TextWrapping="NoWrap"/>
+        </ScrollViewer>
+
+      </Grid>
 
     </DockPanel>
   </Border>

src/ClaudeDo.Ui/Views/Modals/WorktreeModalView.axaml.cs

@@ -21,6 +21,18 @@ public partial class WorktreeModalView : Window
         base.OnDataContextChanged(e);
         if (DataContext is WorktreeModalViewModel vm)
             vm.CloseAction = Close;
+
+        // Wire TreeView selection — SelectedItem TwoWay binding may not fire
+        // reliably in Avalonia 12 for TreeView; use SelectionChanged as backup.
+        var tree = this.FindControl<TreeView>("FileTree");
+        if (tree is not null)
+            tree.SelectionChanged += OnFileTreeSelectionChanged;
+    }
+
+    private void OnFileTreeSelectionChanged(object? sender, SelectionChangedEventArgs e)
+    {
+        if (DataContext is WorktreeModalViewModel vm && sender is TreeView tree)
+            vm.SelectedNode = tree.SelectedItem as WorktreeNodeViewModel;
     }
 
     protected override async void OnOpened(EventArgs e)

src/ClaudeDo.Ui/Views/Modals/WorktreesOverviewModalView.axaml

@@ -8,7 +8,7 @@
         Width="900" Height="560" MinWidth="640" MinHeight="360"
         CanResize="True"
         WindowStartupLocation="CenterOwner"
-        Background="{DynamicResource VoidBrush}"
+        Background="{DynamicResource SurfaceBrush}"
         SystemDecorations="None"
         ExtendClientAreaToDecorationsHint="True">
 
@@ -104,10 +104,7 @@
     </Style>
   </Window.Styles>
 
-  <Border Background="{DynamicResource SurfaceBrush}"
-          BorderBrush="{DynamicResource LineBrush}"
-          BorderThickness="1">
-    <Grid RowDefinitions="36,Auto,*,52">
+  <Grid RowDefinitions="36,Auto,*,52">
 
       <!-- Title bar -->
       <Border Grid.Row="0"
@@ -208,5 +205,4 @@
       </Border>
 
     </Grid>
-  </Border>
 </Window>