feat(ui): coloured dots for icon-less lists, guide line under chevron

Lists without an icon show a dot in a stable identity colour (same
palette/hash as Mission Control tiles); "Dot" is the new picker default.

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

@@ -1,4 +1,5 @@
 using ClaudeDo.Data.Models;
+using ClaudeDo.Ui.ViewModels.MissionControl;
 using CommunityToolkit.Mvvm.ComponentModel;
 
 namespace ClaudeDo.Ui.ViewModels.Islands;
@@ -30,7 +31,15 @@ public sealed partial class ListNavItemViewModel : ViewModelBase
     // DropHintAbove/Below, which mark an insertion point for list reordering.
     [ObservableProperty] private bool _isTaskDropTarget;
     // Settable (not init) so a list-settings icon change refreshes the row in place.
-    [ObservableProperty] private string? _iconKey;
+    [ObservableProperty]
+    [NotifyPropertyChangedFor(nameof(IsDot))]
+    private string? _iconKey;
+
+    /// <summary>No icon picked — the row shows a coloured dot instead of a glyph.</summary>
+    public bool IsDot => IconKey is null or ListsIslandViewModel.ListIcon;
+
+    /// <summary>Identity colour of the dot, stable per list id across restarts.</summary>
+    public string DotColorKey => $"Identity{ConPtyPaneViewModel.StableIndex(Id, 6) + 1}";
     // The sidebar folder this row sits in (ListEntity.FolderId); null = top level.
     [ObservableProperty] private string? _folderId;
 }