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/Controls/DiffLinesView.axaml.cs

@@ -0,0 +1,19 @@
+using System.Collections;
+using Avalonia;
+using Avalonia.Controls;
+
+namespace ClaudeDo.Ui.Views.Controls;
+
+public partial class DiffLinesView : UserControl
+{
+    public static readonly StyledProperty<IEnumerable?> LinesProperty =
+        AvaloniaProperty.Register<DiffLinesView, IEnumerable?>(nameof(Lines));
+
+    public IEnumerable? Lines
+    {
+        get => GetValue(LinesProperty);
+        set => SetValue(LinesProperty, value);
+    }
+
+    public DiffLinesView() => InitializeComponent();
+}