switch storage folder to query param
this was necessary because Windows-style absolute paths don't work well as URL paths.
absolute paths on Windows don't have a leading slash. So we either get:
/storage/folders/addC:/foo/bar
which is obviously invalid, or we manually add the slash:
/storage/folders/add/C:/foo/bar
which works, but now the API sees the path as /C:/foo/bar, which isn't valid. Using a query param seems like the simplest option.