refactor(ui): share color-coded diff rendering between per-task and combined diff viewers

Extract the unified-diff parser into UnifiedDiffParser and the styled line
renderer into a reusable DiffLinesView control. The combined (planning) diff
now parses its unified-diff string and renders color-coded rows (green
additions / red deletions, file headers) identical to the per-task viewer
instead of dumping plain text into a TextBox.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -66,14 +66,7 @@
         <Grid Grid.Column="1" Background="{DynamicResource VoidBrush}">
           <ScrollViewer HorizontalScrollBarVisibility="Auto"
                         VerticalScrollBarVisibility="Auto">
-            <TextBox Text="{Binding DisplayedDiff, Mode=OneWay}"
-                     IsReadOnly="True"
-                     AcceptsReturn="True"
-                     FontFamily="{DynamicResource MonoFont}"
-                     FontSize="{StaticResource FontSizeBody}"
-                     Background="Transparent"
-                     BorderThickness="0"
-                     Padding="8"/>
+            <ctl:DiffLinesView Lines="{Binding DiffLines}"/>
           </ScrollViewer>
         </Grid>