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.Tests/Views/ConverterTests.cs

@@ -0,0 +1,67 @@
+using System.Globalization;
+using PersonalToolBox.Views;
+
+namespace PersonalToolBox.Tests.Views;
+
+public class ConverterTests
+{
+    [Fact]
+    public void BoolToOpacityConverter_True_ReturnsOne()
+    {
+        var converter = new BoolToOpacityConverter();
+
+        var result = converter.Convert(true, typeof(double), null, CultureInfo.InvariantCulture);
+
+        Assert.Equal(1.0, result);
+    }
+
+    [Fact]
+    public void BoolToOpacityConverter_False_ReturnsZeroPointFour()
+    {
+        var converter = new BoolToOpacityConverter();
+
+        var result = converter.Convert(false, typeof(double), null, CultureInfo.InvariantCulture);
+
+        Assert.Equal(0.4, result);
+    }
+
+    [Fact]
+    public void BoolToOpacityConverter_NonBool_ReturnsZeroPointFour()
+    {
+        var converter = new BoolToOpacityConverter();
+
+        var result = converter.Convert("not a bool", typeof(double), null, CultureInfo.InvariantCulture);
+
+        Assert.Equal(0.4, result);
+    }
+
+    [Fact]
+    public void FirstCharConverter_NormalString_ReturnsFirstChar()
+    {
+        var converter = new FirstCharConverter();
+
+        var result = converter.Convert("Postman", typeof(string), null, CultureInfo.InvariantCulture);
+
+        Assert.Equal("P", result);
+    }
+
+    [Fact]
+    public void FirstCharConverter_EmptyString_ReturnsQuestionMark()
+    {
+        var converter = new FirstCharConverter();
+
+        var result = converter.Convert("", typeof(string), null, CultureInfo.InvariantCulture);
+
+        Assert.Equal("?", result);
+    }
+
+    [Fact]
+    public void FirstCharConverter_Null_ReturnsQuestionMark()
+    {
+        var converter = new FirstCharConverter();
+
+        var result = converter.Convert(null!, typeof(string), null, CultureInfo.InvariantCulture);
+
+        Assert.Equal("?", result);
+    }
+}