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/ViewModels/Modals/WorktreeModalViewModel.cs

@@ -10,6 +10,7 @@ public sealed partial class WorktreeNodeViewModel : ViewModelBase
     public required string Name { get; init; }
     public string? Status { get; init; }
     public bool IsDirectory { get; init; }
+    public string RelativePath { get; init; } = "";
     public ObservableCollection<WorktreeNodeViewModel> Children { get; } = new();
 }
 
@@ -21,6 +22,8 @@ public sealed partial class WorktreeModalViewModel : ViewModelBase
 
     [ObservableProperty] private string _worktreePath = "";
     [ObservableProperty] private string? _baseCommit;
+    [ObservableProperty] private WorktreeNodeViewModel? _selectedNode;
+    [ObservableProperty] private string _selectedFileDiff = "";
 
     // Set by the view (same pattern as DiffModalViewModel.CloseAction)
     public Action? CloseAction { get; set; }
@@ -30,6 +33,29 @@ public sealed partial class WorktreeModalViewModel : ViewModelBase
         _git = git;
     }
 
+    partial void OnSelectedNodeChanged(WorktreeNodeViewModel? value)
+    {
+        _ = LoadFileDiffAsync(value);
+    }
+
+    private async Task LoadFileDiffAsync(WorktreeNodeViewModel? node)
+    {
+        if (node is null || node.IsDirectory || string.IsNullOrEmpty(node.RelativePath))
+        {
+            SelectedFileDiff = "";
+            return;
+        }
+
+        try
+        {
+            SelectedFileDiff = await _git.GetFileDiffAsync(WorktreePath, BaseCommit, node.RelativePath);
+        }
+        catch
+        {
+            SelectedFileDiff = "";
+        }
+    }
+
     [RelayCommand]
     private void Close() => CloseAction?.Invoke();
 
@@ -97,7 +123,8 @@ public sealed partial class WorktreeModalViewModel : ViewModelBase
             {
                 Name = segments[^1],
                 Status = status,
-                IsDirectory = false
+                IsDirectory = false,
+                RelativePath = path
             };
             if (parent == null) Root.Add(leaf);
             else parent.Children.Add(leaf);