feat(mui): migrate Login (auth shell + 2FA) onto MUI kit; drop login.css

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/AdminApp.tsx

@@ -20,7 +20,6 @@ import "./datepicker.css";
 import "./filemanager.css";
 import "./pagination.css";
 import "./responsive.css";
-import "./login.css";
 import "./dashboard.css";
 import "./attendance.css";
 import "./plan.css";