feat: implement admin panel functionality in Mini App

- Added new API endpoints for admin features: `GET /api/admin/me`, `GET /api/admin/users`, and `PATCH /api/admin/duties/:id` to manage user duties.
- Introduced `UserForAdmin` and `AdminDutyReassignBody` schemas for handling admin-related data.
- Updated documentation to include Mini App design guidelines and admin panel functionalities.
- Enhanced tests for admin API to ensure proper access control and functionality.
- Improved error handling and localization for admin actions.

.cursor/rules/frontend.mdc

@@ -50,4 +50,15 @@ The Mini App lives in `webapp-next/`. It is built as a static export and served
 - **Run:** `cd webapp-next && npm test` (or `npm run test`). Build: `npm run build`.
 - **Coverage:** Unit tests for lib (api, date-utils, calendar-data, i18n, etc.) and component tests for calendar, duty list, day detail, current duty, states.
 
+## Design guideline
+
+When adding or changing UI in the Mini App, **follow the [Mini App design guideline](../../docs/miniapp-design.md)**:
+
+- Use only design tokens from `globals.css` and Tailwind/shadcn aliases (no hardcoded colours).
+- Page wrappers: `content-safe`, `max-w-[var(--max-width-app)]`, viewport height; respect safe area for sheets/modals.
+- Reuse component patterns (buttons, cards, calendar grid, timeline list) and left-stripe semantics (`border-l-duty`, `border-l-today`, etc.).
+- Add ARIA labels and roles for interactive elements and grids; respect `prefers-reduced-motion` and `data-perf="low"` for animations.
+
+Use the checklist in the design doc when introducing new screens or components.
+
 Consider these rules when changing the Mini App or adding frontend features.