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/nginx.conf

@@ -0,0 +1,46 @@
+server {
+    listen 80;
+    server_name _;
+
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # 前端静态资源
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+
+    # REST API 反代
+    location /api/ {
+        proxy_pass http://backend:8080;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+    }
+
+    # WebSocket 反代
+    location /ws {
+        proxy_pass http://backend:8080;
+        proxy_http_version 1.1;
+        proxy_set_header Upgrade $http_upgrade;
+        proxy_set_header Connection "upgrade";
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+        proxy_read_timeout 86400s;
+        proxy_send_timeout 86400s;
+    }
+
+    # 静态资源缓存
+    location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg|woff|woff2|ttf|eot)$ {
+        expires 7d;
+        add_header Cache-Control "public, immutable";
+    }
+
+    # Gzip 压缩
+    gzip on;
+    gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;
+    gzip_min_length 256;
+}