feat(ui): tasks island with rows, chips, add-task, selection

TaskRowView with status chip (EqStatus converter + parameter),
StrikeIfTrue, NotNullToBool converters. TasksIslandView with header,
add-task TextBox (Enter=AddCommand), ItemsControl + flat Button for
selection. Converters registered in App.axaml.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Ui/Converters/NotNullToBoolConverter.cs

@@ -0,0 +1,15 @@
+using System.Globalization;
+using Avalonia.Data.Converters;
+
+namespace ClaudeDo.Ui.Converters;
+
+public class NotNullToBoolConverter : IValueConverter
+{
+    public static NotNullToBoolConverter Instance { get; } = new();
+
+    public object Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => value is not null;
+
+    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => throw new NotSupportedException();
+}