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/WorktreeModalView.axaml.cs

@@ -21,6 +21,18 @@ public partial class WorktreeModalView : Window
         base.OnDataContextChanged(e);
         if (DataContext is WorktreeModalViewModel vm)
             vm.CloseAction = Close;
+
+        // Wire TreeView selection — SelectedItem TwoWay binding may not fire
+        // reliably in Avalonia 12 for TreeView; use SelectionChanged as backup.
+        var tree = this.FindControl<TreeView>("FileTree");
+        if (tree is not null)
+            tree.SelectionChanged += OnFileTreeSelectionChanged;
+    }
+
+    private void OnFileTreeSelectionChanged(object? sender, SelectionChangedEventArgs e)
+    {
+        if (DataContext is WorktreeModalViewModel vm && sender is TreeView tree)
+            vm.SelectedNode = tree.SelectedItem as WorktreeNodeViewModel;
     }
 
     protected override async void OnOpened(EventArgs e)