feat: 添加前端 Dockerfile 和 Nginx 配置

- 多阶段构建：node:22-alpine 构建，nginx:stable-alpine 运行
- Nginx 配置：静态文件服务 + /api /ws 反代到后端

frontend/Dockerfile

@@ -0,0 +1,25 @@
+# ---- 构建阶段 ----
+FROM node:22-alpine AS builder
+
+WORKDIR /app
+
+# 先复制依赖清单，利用 Docker 缓存层
+COPY package.json package-lock.json ./
+RUN npm ci
+
+# 复制源码并构建
+COPY . .
+RUN npm run build
+
+# ---- 运行阶段 ----
+FROM nginx:stable-alpine
+
+# 复制 Nginx 配置
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+
+# 复制构建产物
+COPY --from=builder /app/dist /usr/share/nginx/html
+
+EXPOSE 80
+
+CMD ["nginx", "-g", "daemon off;"]