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.

AGENTS.md

@@ -24,6 +24,7 @@ Duty Teller is a Telegram bot plus Mini App for team duty shift calendar and gro
 | Duty-schedule parser | `duty_teller/importers/` |
 | Config (env vars) | `duty_teller/config.py` |
 | Miniapp frontend | `webapp-next/` (Next.js, Tailwind, shadcn/ui; static export in `webapp-next/out/`) |
+| Admin panel (Mini App) | `webapp-next/src/app/admin/page.tsx`; API: `GET /api/admin/me`, `GET /api/admin/users`, `PATCH /api/admin/duties/:id` |
 | Migrations | `alembic/` (config in `pyproject.toml` under `[tool.alembic]`) |
 
 ## Running and testing
@@ -35,6 +36,7 @@ Duty Teller is a Telegram bot plus Mini App for team duty shift calendar and gro
 ## Documentation
 
 - User and architecture docs: [docs/](docs/), [docs/architecture.md](docs/architecture.md).
+- [Mini App design guideline](docs/miniapp-design.md) — Theme, layout, safe areas, component patterns, accessibility for webapp-next.
 - Configuration reference: [docs/configuration.md](docs/configuration.md).
 - Build docs: `pip install -e ".[docs]"`, then `mkdocs build` / `mkdocs serve`.
 
@@ -48,4 +50,5 @@ Docstrings and code comments must be in English (Google-style docstrings). UI st
 - **Config:** Environment variables (e.g. `.env`); no hardcoded secrets.
 - **Database:** One logical transaction per `session_scope` — a single `commit` at the end of the business operation (e.g. in `run_import`). Repository helpers used inside such a flow (e.g. `get_or_create_user_by_full_name`) accept `commit=False` and let the caller commit once.
 - **Error handling:** Do not send `str(exception)` from parsers or DB to the user. Use generic i18n keys (e.g. `import.parse_error_generic`, `import.import_error_generic`) and log the full exception server-side.
+- **Mini App (webapp-next):** When adding or changing UI in `webapp-next/`, follow the [Mini App design guideline](docs/miniapp-design.md): use only design tokens and Tailwind aliases from it, apply layout/safe-area/accessibility rules, and use the checklist for new screens or components.
 - **Cursor:** The project does not version `.cursor/`. You can mirror this file in `.cursor/rules/` locally; [AGENTS.md](AGENTS.md) is the single versioned reference for AI and maintainers.