fix(ui): use BorderOnly chrome; color diff +/- lines

Apply SystemDecorations=BorderOnly + ExtendClientAreaTitleBarHeightHint=-1
to WorktreesOverviewModalView and WorktreeModalView for reliable OS resize
borders. Replace SelectedFileDiff SelectableTextBlock with per-line
ItemsControl using WorktreeDiffLineKind coloring via DiffLineKindToBrushConverter.

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

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

@@ -1,18 +1,24 @@
 <Window xmlns="https://github.com/avaloniaui"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:vm="using:ClaudeDo.Ui.ViewModels.Modals"
+        xmlns:converters="using:ClaudeDo.Ui.Converters"
         x:Class="ClaudeDo.Ui.Views.Modals.WorktreeModalView"
         x:DataType="vm:WorktreeModalViewModel"
         Title="Worktree"
         Width="1100" Height="720"
         MinWidth="640" MinHeight="400"
         WindowStartupLocation="CenterOwner"
-        SystemDecorations="None"
+        SystemDecorations="BorderOnly"
         ExtendClientAreaToDecorationsHint="True"
+        ExtendClientAreaTitleBarHeightHint="-1"
         Background="Transparent"
         CanResize="True"
         TransparencyLevelHint="AcrylicBlur">
 
+  <Window.Resources>
+    <converters:DiffLineKindToBrushConverter x:Key="DiffLineKindToBrush"/>
+  </Window.Resources>
+
   <Window.KeyBindings>
     <KeyBinding Gesture="Escape" Command="{Binding CloseCommand}"/>
   </Window.KeyBindings>
@@ -77,11 +83,17 @@
                       HorizontalScrollBarVisibility="Auto"
                       VerticalScrollBarVisibility="Auto"
                       Margin="4,0,8,8">
-          <SelectableTextBlock Text="{Binding SelectedFileDiff}"
-                               FontFamily="{DynamicResource MonoFont}"
-                               FontSize="11"
-                               Foreground="{DynamicResource TextBrush}"
-                               TextWrapping="NoWrap"/>
+          <ItemsControl ItemsSource="{Binding SelectedFileDiffLines}">
+            <ItemsControl.ItemTemplate>
+              <DataTemplate DataType="vm:WorktreeDiffLineViewModel">
+                <SelectableTextBlock Text="{Binding Text}"
+                                     FontFamily="{DynamicResource MonoFont}"
+                                     FontSize="11"
+                                     Foreground="{Binding Kind, Converter={StaticResource DiffLineKindToBrush}}"
+                                     TextWrapping="NoWrap"/>
+              </DataTemplate>
+            </ItemsControl.ItemTemplate>
+          </ItemsControl>
         </ScrollViewer>
 
       </Grid>