refactor(diff): drop rendering helpers orphaned by DiffLinesView

src/ClaudeDo.Ui/Design/IslandStyles.axaml

@@ -1111,13 +1111,6 @@
     <Setter Property="Background"      Value="{StaticResource DeepBrush}" />
   </Style>
 
-  <!-- Diff line-number gutter column -->
-  <Style Selector="TextBlock.diff-lineno">
-    <Setter Property="FontFamily" Value="{StaticResource MonoFont}" />
-    <Setter Property="FontSize"   Value="{StaticResource FontSizeMono}" />
-    <Setter Property="Foreground" Value="{StaticResource TextFaintBrush}" />
-  </Style>
-
   <!-- Terminal selectable log text (SelectableTextBlock doesn't inherit the TextBlock terminal style) -->
   <Style Selector="Border.terminal SelectableTextBlock">
     <Setter Property="FontFamily" Value="{StaticResource MonoFont}" />

src/ClaudeDo.Ui/ViewModels/Modals/DiffModels.cs

@@ -15,20 +15,6 @@ public sealed class DiffLineViewModel
     public int? OldNo { get; init; }
     public int? NewNo { get; init; }
     public required string Text { get; init; }
-    public string ClassName => Kind switch
-    {
-        DiffLineKind.Add  => "add",
-        DiffLineKind.Del  => "del",
-        DiffLineKind.File => "file",
-        _                 => "ctx",
-    };
-
-    public string Sign => Kind switch
-    {
-        DiffLineKind.Add => "+",
-        DiffLineKind.Del => "-",
-        _                => " ",
-    };
 }
 
 public sealed class DiffFileViewModel