Phase 2-3: UI layout, theme switching, CRUD tools, process execution

- Phase 2: MainWindow 3-section layout (sidebar/content/log bar), Dark/Light theme with ThemeHelper, MainViewModel with ObservableProperty/RelayCommand, tool card filtering by search + category

- Phase 3: ToolEditWindow for add/edit tools, ProcessExecutionService (Process.Start + error handling), double-click + right-click context menu (run/edit), path browse dialog

- Bugfix: ContextMenu commands now use PlacementTarget.Tag binding (ContextMenu in separate visual tree)

- Bugfix: StaticResource converters moved to XAML before DataTemplate to fix XamlParseException on tool card render

- Bugfix: Pure filenames (no path separators) treated as PATH commands, not marked invalid

- Bugfix: RefreshData preserves SelectedCategory; Load() catches all exceptions; Save() wrapped in try-catch; auto-scroll log to newest entry

- Tests: xUnit project with 55 tests covering models, services, converters, and view models

PersonalToolBox/Helpers/ThemeHelper.cs

@@ -0,0 +1,55 @@
+using System;
+using System.Collections.ObjectModel;
+using System.Windows;
+
+namespace PersonalToolBox.Helpers;
+
+/// <summary>
+/// 主题切换辅助类，动态替换 Application 的 ResourceDictionary
+/// </summary>
+public static class ThemeHelper
+{
+    private const string ThemePrefix = "Themes/";
+    private const string DarkThemePath = "Themes/DarkTheme.xaml";
+    private const string LightThemePath = "Themes/LightTheme.xaml";
+
+    /// <summary>
+    /// 切换主题（Dark / Light）
+    /// </summary>
+    public static void ApplyTheme(string theme)
+    {
+        var app = Application.Current;
+        if (app == null) return;
+
+        // 移除旧的主题资源字典
+        var oldDict = FindThemeDictionary(app.Resources.MergedDictionaries);
+        if (oldDict != null)
+            app.Resources.MergedDictionaries.Remove(oldDict);
+
+        // 加载新的主题资源字典
+        var path = theme switch
+        {
+            "Light" => LightThemePath,
+            _ => DarkThemePath
+        };
+
+        var newDict = new ResourceDictionary
+        {
+            Source = new Uri(path, UriKind.Relative)
+        };
+        app.Resources.MergedDictionaries.Add(newDict);
+    }
+
+    /// <summary>
+    /// 在当前合并字典中查找主题相关的 ResourceDictionary
+    /// </summary>
+    private static ResourceDictionary? FindThemeDictionary(Collection<ResourceDictionary> dictionaries)
+    {
+        foreach (var dict in dictionaries)
+        {
+            if (dict.Source != null && dict.Source.OriginalString.StartsWith(ThemePrefix))
+                return dict;
+        }
+        return null;
+    }
+}