handle_directory_rename updated Folder.path but left Folder.name as
the old leaf basename. Path is load-bearing; name is purely display,
but a stale name shows wrong text in the sidebar tree until the next
manual refresh. Now sets folder.name = basename(new_prefix) on the
renamed folder itself; descendants keep their existing names because
the rename was on an ancestor (only their paths shift).
Same correctness as the existing PATCH /folders/{id} endpoint, which
also updates both name and path.
f4618ddf97