feat(lists): allow nested sidebar folders via drag and drop

Folders can now contain other folders, to arbitrary depth, via a
self-referencing parent_folder_id (ON DELETE SET NULL, same pattern as
lists.folder_id). Nesting/un-nesting is drag-and-drop only: dropping a
folder onto another folder's header middle moves it in, the outer
quarters or any top-level/in-folder drop insert it beside a target.
Dropping a folder into itself or one of its own subfolders is silently
ignored. Deleting a folder releases its lists and subfolders back to
the top level, unchanged from the existing list behavior.

ListFolderNavItemViewModel.Lists became Children (object, mixing lists
and subfolders, mirroring TopLevel) so the existing implicit, type-matched
DataTemplates render nested folders recursively without new XAML, and each
nesting level gets its indent for free from the folder template's own
margin. Folders/RootLists/ContainerOf/PersistOrderAsync/RecomputeFolderCounts
now walk the tree instead of assuming one level.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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

@@ -139,7 +139,7 @@
             </Grid>
           </Border>
 
-          <ItemsControl ItemsSource="{Binding Lists}"
+          <ItemsControl ItemsSource="{Binding Children}"
                         IsVisible="{Binding !IsCollapsed}"
                         Margin="14,0,0,0"/>
         </StackPanel>

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

@@ -142,18 +142,21 @@ public partial class ListsIslandView : UserControl
     private enum FolderDrop { None, Into, Beside }
 
     /// <summary>
-    /// What a drop on a folder header means. For a list the middle half moves it *into* the folder
+    /// What a drop on a folder header means: the middle half moves the source *into* the folder
     /// (the only route into a collapsed or empty one) while the outer quarters insert next to the
-    /// folder at the top level; a folder source has no "into" — folders never nest — so it splits
-    /// in half.
+    /// folder in its own container. A folder dropped onto itself or one of its own subfolders has
+    /// no valid "into" or "beside" and is rejected.
     /// </summary>
     private static (FolderDrop Kind, bool PlaceBelow) ClassifyFolderDrop(
         object source, ListFolderNavItemViewModel folder, double y, double height)
     {
         if (source is ListFolderNavItemViewModel dragged)
-            return ReferenceEquals(dragged, folder)
-                ? (FolderDrop.None, false)
-                : (FolderDrop.Beside, y > height / 2);
+        {
+            if (IsSelfOrDescendant(dragged, folder)) return (FolderDrop.None, false);
+            if (y < height * 0.25) return (FolderDrop.Beside, false);
+            if (y > height * 0.75) return (FolderDrop.Beside, true);
+            return dragged.ParentFolderId == folder.Id ? (FolderDrop.None, false) : (FolderDrop.Into, false);
+        }
 
         if (source is not ListNavItemViewModel row) return (FolderDrop.None, false);
         if (y < height * 0.25) return (FolderDrop.Beside, false);
@@ -161,6 +164,14 @@ public partial class ListsIslandView : UserControl
         return row.FolderId == folder.Id ? (FolderDrop.None, false) : (FolderDrop.Into, false);
     }
 
+    private static bool IsSelfOrDescendant(ListFolderNavItemViewModel folder, ListFolderNavItemViewModel candidate)
+    {
+        if (ReferenceEquals(folder, candidate)) return true;
+        foreach (var child in folder.Children.OfType<ListFolderNavItemViewModel>())
+            if (IsSelfOrDescendant(child, candidate)) return true;
+        return false;
+    }
+
     private void OnFolderDragOver(object? sender, DragEventArgs e)
     {
         if (DataContext is not ListsIslandViewModel vm) { e.DragEffects = DragDropEffects.None; return; }
@@ -201,8 +212,8 @@ public partial class ListsIslandView : UserControl
 
             var (kind, placeBelow) = ClassifyFolderDrop(source, folder, e.GetPosition(b).Y, b.Bounds.Height);
             vm.ClearDropHints();
-            if (kind == FolderDrop.Into && source is ListNavItemViewModel row)
-                await vm.MoveToFolderAsync(row, folder);
+            if (kind == FolderDrop.Into)
+                await vm.MoveToFolderAsync(source, folder);
             else if (kind == FolderDrop.Beside)
                 await vm.DropAsync(source, folder, placeBelow);
         }
@@ -224,8 +235,6 @@ public partial class ListsIslandView : UserControl
 
         var source = DragSource(vm, e);
         if (source is null || ReferenceEquals(source, target)) { Reject(vm, e); return; }
-        // A folder can only be dropped at the top level, so a row inside a folder is no target.
-        if (source is ListFolderNavItemViewModel && target.FolderId is not null) { Reject(vm, e); return; }
 
         // Canonicalize: "drop below X" == "drop above X+1". Only the last row shows a below-line.
         object hintTarget = target;
@@ -251,7 +260,6 @@ public partial class ListsIslandView : UserControl
 
             var source = DragSource(vm, e);
             if (source is null || ReferenceEquals(source, target)) return;
-            if (source is ListFolderNavItemViewModel && target.FolderId is not null) return;
 
             var placeBelow = e.GetPosition(b).Y > b.Bounds.Height / 2;
             vm.ClearDropHints();