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.

webapp-next/src/components/admin/index.ts

@@ -0,0 +1,9 @@
+/**
+ * Admin feature: hook and presentational components for the admin page.
+ */
+
+export { useAdminPage } from "./useAdminPage";
+export { AdminDutyList } from "./AdminDutyList";
+export { ReassignSheet } from "./ReassignSheet";
+export type { AdminDutyListProps } from "./AdminDutyList";
+export type { ReassignSheetProps } from "./ReassignSheet";