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/ViewModels/Islands/TasksIslandViewModel.cs

@@ -33,7 +33,9 @@ public sealed partial class TasksIslandViewModel : ViewModelBase
     [ObservableProperty] private bool _hasStatusPill;
     [ObservableProperty] private bool _isShowingCompleted = true;
     [ObservableProperty] private bool _hasOverdue;
+    [ObservableProperty] private bool _hasOpen;
     [ObservableProperty] private bool _hasCompleted;
+    [ObservableProperty] private bool _showOpenLabel;
     [ObservableProperty] private string _completedHeader = "COMPLETED";
 
     public TasksIslandViewModel(IDbContextFactory<ClaudeDoDbContext> dbFactory)
@@ -53,8 +55,10 @@ public sealed partial class TasksIslandViewModel : ViewModelBase
         OverdueItems.Clear();
         OpenItems.Clear();
         CompletedItems.Clear();
-        HasOverdue = false;
-        HasCompleted = false;
+        HasOverdue    = false;
+        HasOpen       = false;
+        HasCompleted  = false;
+        ShowOpenLabel = false;
         if (list is null) return;
 
         HeaderTitle = list.Name;
@@ -112,8 +116,10 @@ public sealed partial class TasksIslandViewModel : ViewModelBase
                 OpenItems.Add(r);
         }
 
-        HasOverdue    = OverdueItems.Count > 0;
-        HasCompleted  = CompletedItems.Count > 0;
+        HasOverdue     = OverdueItems.Count > 0;
+        HasOpen        = OpenItems.Count > 0;
+        HasCompleted   = CompletedItems.Count > 0;
+        ShowOpenLabel  = HasOpen && HasOverdue;
         CompletedHeader = $"COMPLETED · {CompletedItems.Count}";
     }