Refactor admin/email-settings to use Composition API (#8656)

* refactor(client): refactor admin/email-settings to use Composition API

* Update packages/client/src/pages/admin/email-settings.vue

Co-authored-by: Johann150 <johann@qwertqwefsday.eu>

Co-authored-by: syuilo <Syuilotan@yahoo.co.jp>
Co-authored-by: Johann150 <johann@qwertqwefsday.eu>