refactor(admin): CRM을 apps/admin 독립 프로젝트로 분리

- apps/web에서 admin 라우트/가드/sidebar 링크 제거
- apps/admin: 독립 Next.js 앱 (포트 3001)
  - 자체 login/unauthorized/auth callback
  - admin-sidebar: Overview/Users/Subscriptions/Usage
  - requireAdmin() 가드: profile.role='admin' 체크
- monorepo workspace에 apps/admin 등록

apps/web/src/components/layout/sidebar.tsx

@@ -25,7 +25,6 @@ import LibraryBooksIcon from '@mui/icons-material/LibraryBooks'
 import AutoAwesomeIcon from '@mui/icons-material/AutoAwesome'
 import GroupsIcon from '@mui/icons-material/Groups'
 import PaymentIcon from '@mui/icons-material/Payment'
-import AdminPanelSettingsIcon from '@mui/icons-material/AdminPanelSettings'
 import LogoutIcon from '@mui/icons-material/Logout'
 import PaletteIcon from '@mui/icons-material/Palette'
 import type { ThemeMode } from '@d3ro/core/types'
@@ -100,12 +99,6 @@ export function Sidebar(): React.ReactElement {
       path: '/billing',
       label: t('nav.billing') ?? 'Billing',
       icon: <PaymentIcon />
-    },
-    {
-      key: 'admin',
-      path: '/admin',
-      label: 'Admin',
-      icon: <AdminPanelSettingsIcon />
     }
   ]