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.

Dockerfile

@@ -1,14 +1,22 @@
-# Multi-stage: builder installs deps; runtime copies only site-packages and app code.
+# Multi-stage: webapp build (Next.js), Python builder, runtime.
 # Single image for both dev and prod; Compose files differentiate behavior.
 
-# --- Stage 1: builder (dependencies only) ---
+# --- Stage 1: webapp build (Next.js static export) ---
+FROM node:20-slim AS webapp-builder
+WORKDIR /webapp
+COPY webapp-next/package.json webapp-next/package-lock.json* ./
+RUN npm ci
+COPY webapp-next/ ./
+RUN npm run build
+
+# --- Stage 2: builder (Python dependencies only) ---
 FROM python:3.12-slim AS builder
 WORKDIR /app
 COPY pyproject.toml ./
 COPY duty_teller/ ./duty_teller/
 RUN pip install --no-cache-dir .
 
-# --- Stage 2: runtime (minimal final image) ---
+# --- Stage 3: runtime (minimal final image) ---
 FROM python:3.12-slim
 
 WORKDIR /app
@@ -27,7 +35,7 @@ COPY main.py pyproject.toml entrypoint.sh ./
 RUN chmod +x entrypoint.sh
 COPY duty_teller/ ./duty_teller/
 COPY alembic/ ./alembic/
-COPY webapp/ ./webapp/
+COPY --from=webapp-builder /webapp/out ./webapp-next/out
 
 # Create data dir; entrypoint runs as root, fixes perms for volume, then runs app as botuser
 RUN adduser --disabled-password --gecos "" botuser \