feat(ui): collapsible folders for the list sidebar

Folders render above the top-level lists, each with its own collapsible
children; tapping a folder header folds it instead of selecting it.
Lists move in by dragging onto a folder header (or onto a row inside it)
and out by dropping between top-level rows or via the context menu.

Creating a folder is a toggle in the New-list modal, not a third sidebar
button: it reduces the modal to name + icon and writes a folder row
instead of the list row the create flow pre-created.

src/ClaudeDo.Ui/Views/Islands/ListsIslandView.axaml.cs

@@ -69,6 +69,60 @@ public partial class ListsIslandView : UserControl
         }
     }
 
+    private void OnFolderTapped(object? sender, RoutedEventArgs e)
+    {
+        if (sender is Border { DataContext: ListFolderNavItemViewModel folder }
+            && DataContext is ListsIslandViewModel vm)
+            vm.ToggleFolderCommand.Execute(folder);
+    }
+
+    // Dropping onto the folder *header* appends the list to that folder — the only route into a
+    // collapsed or empty folder, where there's no child row to drop next to.
+    private void OnFolderDragOver(object? sender, DragEventArgs e)
+    {
+        if (DataContext is not ListsIslandViewModel vm) { e.DragEffects = DragDropEffects.None; return; }
+        if ((!e.DataTransfer?.Contains(ListRowFormat) ?? true)
+            || sender is not Border { DataContext: ListFolderNavItemViewModel folder })
+        {
+            e.DragEffects = DragDropEffects.None;
+            vm.ClearDropHints();
+            return;
+        }
+
+        var sourceId = e.DataTransfer?.TryGetValue(ListRowFormat);
+        var source = sourceId is null ? null : vm.UserLists.FirstOrDefault(r => r.Id == sourceId);
+        if (source is null || source.FolderId == folder.Id)
+        {
+            e.DragEffects = DragDropEffects.None;
+            vm.ClearDropHints();
+            return;
+        }
+
+        vm.SetFolderDropTarget(folder);
+        e.DragEffects = DragDropEffects.Move;
+    }
+
+    private async void OnFolderDrop(object? sender, DragEventArgs e)
+    {
+        if (DataContext is not ListsIslandViewModel vm) return;
+        try
+        {
+            if (sender is not Border { DataContext: ListFolderNavItemViewModel folder }) return;
+
+            var sourceId = e.DataTransfer?.TryGetValue(ListRowFormat);
+            var source = sourceId is null ? null : vm.UserLists.FirstOrDefault(r => r.Id == sourceId);
+            if (source is null) return;
+
+            vm.ClearDropHints();
+            await vm.MoveToFolderAsync(source, folder);
+        }
+        catch
+        {
+            vm.ClearDropHints();
+            throw;
+        }
+    }
+
     private void OnListDragOver(object? sender, DragEventArgs e)
     {
         if (DataContext is not ListsIslandViewModel vm) { e.DragEffects = DragDropEffects.None; return; }
@@ -150,11 +204,14 @@ public partial class ListsIslandView : UserControl
         return null;
     }
 
+    // Next row *within the same container* — the row below the last list of a folder belongs to
+    // another folder (or to the top level), so it is not a "drop above" candidate here.
     private static ListNavItemViewModel? FindNextUserList(ListsIslandViewModel vm, ListNavItemViewModel row)
     {
-        var idx = vm.UserLists.IndexOf(row);
+        var container = vm.ContainerOf(row);
+        var idx = container.IndexOf(row);
         if (idx < 0) return null;
-        return idx + 1 < vm.UserLists.Count ? vm.UserLists[idx + 1] : null;
+        return idx + 1 < container.Count ? container[idx + 1] : null;
     }
 
 }