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.

docs/index.md

@@ -6,6 +6,7 @@ Telegram bot for team duty shift calendar and group reminder. The bot and web UI
 
 - [Configuration](configuration.md) — Environment variables (types, defaults, examples).
 - [Architecture](architecture.md) — Components, data flow, package relationships.
+- [Mini App design](miniapp-design.md) — Design guideline for the Telegram Mini App (webapp-next): theme, layout, components, accessibility.
 - [Import format](import-format.md) — Duty-schedule JSON format and example.
 - [Runbook](runbook.md) — Running the app, logs, common errors, DB and migrations.
 - [API Reference](api-reference.md) — Generated from code (api, db, services, handlers, importers, config).