fix(frontend): serve dist via nginx instead of vite preview

User reported nginx upstream connect refused for frontend:3000.
vite preview is dev/preview-oriented and has been observed dropping
its listener in docker production environments.

- Frontend Dockerfile: multi-stage build → nginx:alpine serves /usr/share/nginx/html
- Frontend nginx.conf: SPA fallback (try_files ... /index.html) so client-side
  routes like /domains survive a browser reload, plus immutable cache for /assets/
- docker-compose: frontend now exposes 80 instead of 3000

Top-level nginx upstream (server frontend:3000) already resolves by service name;
port mapping in upstream is unaffected because http upstream uses the resolved
address and the upstream block targets the container's listening port. Updating
to frontend:80 happens automatically because the upstream uses the service name
without an explicit port override.

Actually correction: the upstream IS port-bound. Updating both ends in one commit.

frontend/Dockerfile

@@ -1,4 +1,4 @@
-# 빌드 단계
+# 빌드 단계: vite 로 dist/ 만든다
 FROM node:20-alpine AS build
 WORKDIR /app
 COPY package.json ./
@@ -6,12 +6,10 @@ RUN npm install --no-audit --no-fund
 COPY . ./
 RUN npm run build
 
-# 서빙 단계: vite preview 로 dist/ 정적 서빙
-FROM node:20-alpine
-WORKDIR /app
-COPY --from=build /app/package.json ./
-COPY --from=build /app/node_modules ./node_modules
-COPY --from=build /app/dist ./dist
-COPY --from=build /app/vite.config.js ./
-EXPOSE 3000
-CMD ["npx", "vite", "preview", "--host", "0.0.0.0", "--port", "3000"]
+# 서빙 단계: nginx 가 정적으로 dist/ 를 서빙한다.
+# (vite preview 는 dev/preview 용이고 docker production 환경에서 연결 끊김
+#  현상이 보고된 적이 있어 nginx 정적 서빙으로 통일)
+FROM nginx:alpine
+COPY --from=build /app/dist /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+EXPOSE 80