feat(ui): make sidebar folder hierarchy readable

Lists default to the Text icon instead of the folder icon, rows align
across depths with a guide line per open folder, folder names are
semibold, folder counts only show while collapsed and zero counts hide.

tests/ClaudeDo.Ui.Tests/ViewModels/ListSettingsModalViewModelTests.cs

@@ -59,6 +59,21 @@ public class ListSettingsModalViewModelTests
         }
     }
 
+    [Fact]
+    public void FolderToggle_SwapsUntouchedDefaultIcon_KeepsCustomChoice()
+    {
+        var vm = new ListSettingsModalViewModel(new CountingWorker(), NewFactory());
+
+        vm.IsFolder = true;
+        Assert.Equal(ListSettingsModalViewModel.DefaultFolderIcon, vm.Icon);
+        vm.IsFolder = false;
+        Assert.Equal(ListSettingsModalViewModel.DefaultIcon, vm.Icon);
+
+        vm.Icon = "Star";
+        vm.IsFolder = true;
+        Assert.Equal("Star", vm.Icon);
+    }
+
     [Fact]
     public async Task LoadAsync_CallsGetListConfig_ExactlyOnce()
     {