refactor(ui): flatten TasksIsland's task list into one virtualized ListBox

Replace the three ItemsControl sections (Overdue/Open/Completed) with a single
flat Rows collection (HeaderRow | TaskRowViewModel) bound to a ListBox +
VirtualizingStackPanel, so the realized container count stays bounded instead
of growing with the list (measured: ~7-8 realized containers at any scroll
position with 1000 source rows). Group headers become regular row entries,
each section is simply omitted from Rows when empty, and the Completed
section's Clear-completed action moves onto the header row itself.

SectionFor/ReorderAsync/FindNextInSameSection now read a row's section off
the nearest preceding HeaderRow in Rows instead of three separate bound
collections, and ScrollSelectedIntoView goes through ListBox.ScrollIntoView so
it still works for rows outside the realized window. Drag/reorder behavior,
grouping/counts, and the Clear-completed button are unchanged.

Auto-scroll-while-dragging and the cross-section reorder bug are explicitly
out of scope (follow-up tasks).

src/ClaudeDo.Ui/Converters/NotHeaderRowConverter.cs

@@ -0,0 +1,18 @@
+using System.Globalization;
+using Avalonia.Data.Converters;
+using ClaudeDo.Ui.ViewModels.Islands;
+
+namespace ClaudeDo.Ui.Converters;
+
+// Drives ListBoxItem.Focusable for TasksIslandView's flat Rows list: a HeaderRow container must
+// not be a tab stop, a TaskRowViewModel container must behave like any other row.
+public class NotHeaderRowConverter : IValueConverter
+{
+    public static NotHeaderRowConverter Instance { get; } = new();
+
+    public object Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => value is not HeaderRow;
+
+    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => throw new NotSupportedException();
+}