feat: migrate to Next.js for Mini App and enhance project structure

- Replaced the previous webapp with a new Mini App built using Next.js, improving performance and maintainability.
- Updated the `.gitignore` to exclude Next.js build artifacts and node modules.
- Revised documentation in `AGENTS.md`, `README.md`, and `architecture.md` to reflect the new Mini App structure and technology stack.
- Enhanced Dockerfile to support the new build process for the Next.js application.
- Updated CI workflow to build and test the Next.js application.
- Added new configuration options for the Mini App, including `MINI_APP_SHORT_NAME` for improved deep linking.
- Refactored frontend testing setup to accommodate the new structure and testing framework.
- Removed legacy webapp files and dependencies to streamline the project.

docs/architecture.md

@@ -5,7 +5,7 @@ High-level architecture of Duty Teller: components, data flow, and package relat
 ## Components
 
 - **Bot** — [python-telegram-bot](https://github.com/python-telegram-bot/python-telegram-bot) v22 (Application API). Handles commands and group messages; runs in polling mode.
-- **FastAPI** — HTTP server: REST API (`/api/duties`, `/api/calendar-events`, `/api/calendar/ical/{token}.ics`) and static miniapp at `/app`. Runs in a separate thread alongside the bot.
+- **FastAPI** — HTTP server: REST API (`/api/duties`, `/api/calendar-events`, `/api/calendar/ical/{token}.ics`) and static miniapp at `/app` (built from `webapp-next/`, Next.js static export). Runs in a separate thread alongside the bot.
 - **Database** — SQLAlchemy ORM with Alembic migrations. Default backend: SQLite (`data/duty_teller.db`). Stores users, duties (with event types: duty, unavailable, vacation), group duty pins, calendar subscription tokens.
 - **Duty-schedule import** — Two-step admin flow: handover time (timezone → UTC), then JSON file. Parser produces per-person date lists; import service deletes existing duties in range and inserts new ones.
 - **Group duty pin** — In groups, the bot can pin the current duty message; time/timezone for the pinned text come from `DUTY_DISPLAY_TZ`. Pin state is restored on startup from the database. When the duty changes on schedule, the bot sends a new message, unpins the previous one and pins the new one; if `DUTY_PIN_NOTIFY` is enabled (default), pinning the new message triggers a Telegram notification for members. The first pin (bot added to group or `/pin_duty`) is always silent.