2.0.21: Drop duplicate /admin-shell/* routes; fix email-settings multi-root

- registerAdminRoutes() no longer registers /admin-shell/config/* and /admin-shell/language-strings: those duplicated the canonical config screens (embedded in SettingsHub) and admin_language_manager.index; 0 references remained.
- Wrap email-settings.blade.php in a single root element so it renders both as a full page and embedded in SettingsHub (Livewire v3 single-root requirement), fixing MultipleRootElementsDetectedException on /store_config.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>