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/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>