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.Data/Configuration/ListFolderEntityConfiguration.cs

@@ -17,7 +17,14 @@ public class ListFolderEntityConfiguration : IEntityTypeConfiguration<ListFolder
         builder.Property(f => f.Icon).HasColumnName("icon");
         builder.Property(f => f.SortOrder).HasColumnName("sort_order").IsRequired().HasDefaultValue(0);
         builder.Property(f => f.IsCollapsed).HasColumnName("is_collapsed").IsRequired().HasDefaultValue(false);
+        builder.Property(f => f.ParentFolderId).HasColumnName("parent_folder_id");
 
         builder.HasIndex(f => f.SortOrder).HasDatabaseName("idx_list_folders_sort");
+        builder.HasIndex(f => f.ParentFolderId).HasDatabaseName("idx_list_folders_parent");
+
+        builder.HasOne<ListFolderEntity>()
+            .WithMany()
+            .HasForeignKey(f => f.ParentFolderId)
+            .OnDelete(DeleteBehavior.SetNull);
     }
 }