feat: enhance admin page functionality with new components and hooks

- Added `AdminDutyList` and `ReassignSheet` components for improved duty management in the admin panel.
- Introduced `useAdminPage` hook to encapsulate admin-related logic, including user and duty loading, and reassign functionality.
- Updated `frontend.mdc` documentation to reflect new admin components and their usage.
- Improved error handling for API responses, particularly for access denied scenarios.
- Refactored admin page to utilize new components, streamlining the UI and enhancing maintainability.

.cursor/rules/frontend.mdc

@@ -27,6 +27,7 @@ The Mini App lives in `webapp-next/`. It is built as a static export and served
 | Duty list | `src/components/duty/` — DutyList, DutyTimelineCard, DutyItem |
 | Day detail | `src/components/day-detail/` — DayDetail (Sheet/Popover), DayDetailContent |
 | Current duty view | `src/components/current-duty/CurrentDutyView.tsx` |
+| Admin | `src/components/admin/` — useAdminPage, AdminDutyList, ReassignSheet |
 | Contact links | `src/components/contact/ContactLinks.tsx` |
 | State views | `src/components/states/` — LoadingState, ErrorState, AccessDenied |
 | Hooks | `src/hooks/` — use-telegram-theme, use-telegram-auth, use-month-data, use-swipe, use-media-query, use-sticky-scroll, use-auto-refresh |
@@ -42,6 +43,7 @@ The Mini App lives in `webapp-next/`. It is built as a static export and served
 - **Auth:** `useTelegramAuth` provides initData for API; access gated for non-Telegram except localhost.
 - **i18n:** `useTranslation()` from store lang; `window.__DT_LANG` set by `/app/config.js` (backend).
 - **API:** `fetchDuties`, `fetchCalendarEvents` in `src/lib/api.ts`; pass initData, lang, AbortSignal; handle ACCESS_DENIED.
+- **Heavy pages:** For feature-heavy routes (e.g. admin), use a custom hook (state, effects, callbacks) plus presentational components; keep the page as a thin layer (early returns + composition). Example: `admin/page.tsx` uses `useAdminPage`, `AdminDutyList`, and `ReassignSheet`.
 
 ## Testing