feat(ui): add About modal opened from Help menu

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,34 @@
+using System.Diagnostics;
+using System.IO;
+using System.Reflection;
+using ClaudeDo.Data;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+
+namespace ClaudeDo.Ui.ViewModels.Modals;
+
+public sealed partial class AboutModalViewModel : ViewModelBase
+{
+    public string AppVersion { get; } =
+        Assembly.GetExecutingAssembly().GetName().Version?.ToString(3) ?? "0.0.0";
+    public string DataFolderPath { get; } = Paths.AppDataRoot();
+    public string LogsFolderPath { get; } = Path.Combine(Paths.AppDataRoot(), "logs");
+    public string WorkerConfigPath { get; } = Path.Combine(Paths.AppDataRoot(), "worker.config.json");
+
+    public Action? CloseAction { get; set; }
+
+    [RelayCommand] private void Close() => CloseAction?.Invoke();
+
+    [RelayCommand]
+    private void OpenPath(string? path)
+    {
+        if (string.IsNullOrWhiteSpace(path)) return;
+        try
+        {
+            var target = File.Exists(path) ? path : (Directory.Exists(path) ? path : null);
+            if (target is null) return;
+            Process.Start(new ProcessStartInfo("explorer.exe", $"\"{target}\"") { UseShellExecute = true });
+        }
+        catch { /* ignore */ }
+    }
+}