fix(ui): clear task selection when switching lists

Switching lists rebuilt Tasks.Items from scratch but left SelectedTask pointing
at a row from the previous list, so the detail pane kept showing that task even
though it is not in the visible list. Drop the selection when the list actually
changes; a reload of the same list (worker refresh, reconnect) keeps it so a
live update never yanks the detail pane away.

src/ClaudeDo.Ui/ViewModels/Islands/TasksIslandViewModel.cs

@@ -231,6 +231,12 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
         _loadCts = new CancellationTokenSource();
         var ct = _loadCts.Token;
 
+        // Items is rebuilt from scratch below, so a selection carried over from the previous list
+        // would leave the detail pane bound to a task the visible list no longer contains. Only a
+        // *different* list drops it — a reload of the same list (worker refresh, reconnect) keeps
+        // the selection so a live update never yanks the detail pane away.
+        var listChanged = !string.Equals(_currentList?.Id, list?.Id, StringComparison.Ordinal);
+
         if (_currentList is not null)
             _currentList.PropertyChanged -= OnCurrentListPropertyChanged;
         _currentList = list;
@@ -246,6 +252,7 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
         HasCompleted  = false;
         ShowOpenLabel = false;
         ShowNotesRow  = false;
+        if (listChanged) SelectedTask = null;
         if (list is null) { IsLetClaudeVisible = false; LoadTask = Task.CompletedTask; return; }
 
         HeaderTitle = list.Name;