style(ui): task section dividers overdue/tasks/completed

Expose OverdueItems / OpenItems / CompletedItems as separate observable
collections recomputed in LoadForList (and after add / toggle-done).

- OverdueItems: ScheduledFor.Date < Today && !Done
- OpenItems:    remaining !Done
- CompletedItems: Done

View renders three sections with eyebrow-style headers:
- OVERDUE (blood accent, only when non-empty)
- TASKS (shown only when overdue is also visible, matching the mock)
- COMPLETED · N (hidden when IsShowingCompleted is false)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/Islands/TasksIslandView.axaml

@@ -89,10 +89,10 @@
         </StackPanel>
 
         <!-- TASKS -->
-        <StackPanel>
+        <StackPanel IsVisible="{Binding HasOpen}">
           <TextBlock Classes="eyebrow section-label"
                      Text="TASKS" Margin="14,14,14,6"
-                     IsVisible="{Binding HasOverdue}"/>
+                     IsVisible="{Binding ShowOpenLabel}"/>
           <ItemsControl ItemsSource="{Binding OpenItems}">
             <ItemsControl.ItemTemplate>
               <DataTemplate DataType="vm:TaskRowViewModel">