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/App.xaml

@@ -3,6 +3,10 @@
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:local="clr-namespace:PersonalToolBox">
     <Application.Resources>
-
+        <ResourceDictionary>
+            <ResourceDictionary.MergedDictionaries>
+                <ResourceDictionary Source="Themes/DarkTheme.xaml"/>
+            </ResourceDictionary.MergedDictionaries>
+        </ResourceDictionary>
     </Application.Resources>
 </Application>