fix: replace broken multi-stage frontend copy with inline npm build

Dockerfile

@@ -149,8 +149,11 @@ EXPOSE 7860
 
 
 
-# TRAVELER_DEV_FRONTEND_RUNTIME_COPY
-# Next.js static export generated by traveler_frontend_builder.
-COPY --from=traveler_frontend_builder /workspace/frontend/out ./frontend/out
+# Build Next.js frontend
+RUN apt-get update && apt-get install -y --no-install-recommends nodejs npm && rm -rf /var/lib/apt/lists/*
+COPY frontend/package.json frontend/package-lock.json ./frontend/
+RUN cd frontend && npm install --legacy-peer-deps
+COPY frontend/ ./frontend/
+RUN cd frontend && npm run build
 
 CMD ["python", "-m", "uvicorn", "app:app", "--host", "0.0.0.0", "--port", "7860"]