fix(ui): re-emit the flat list's section headers on a language change

Phase 2b-1 moved the group headers from {loc:Tr} bindings in TasksIslandView
into HeaderRow.Label, resolved once in Regroup(). RefreshLocalizedText only
updated CompletedHeader and the task rows, so after switching the language the
three section headers kept the old one. Re-run Regroup() there.

Not unit-tested: the only seam is the global static Loc.Current, and mutating
it would bleed into test classes running in parallel.

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

@@ -165,6 +165,9 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
     {
         CompletedHeader = Loc.T("vm.tasksIsland.completedHeader");
         foreach (var row in Items) row.RefreshLocalized();
+        // The section headers are now Rows entries whose Label is a resolved string, not a live
+        // {loc:Tr} binding in the view — without re-emitting them they keep the old language.
+        Regroup();
     }
 
     private async void OnWorkerListUpdated(string listId)