📦 添加虚拟环境和启动脚本

新增:
- backend/venv/ - Python 虚拟环境
- backend/start.sh - 启动脚本（使用虚拟环境）
- backend/requirements.txt - 依赖列表
- .gitignore - 忽略虚拟环境和缓存文件

说明:
- 每个项目使用独立虚拟环境
- 避免依赖冲突
- 启动脚本自动创建和激活虚拟环境

frontend/node_modules/default-gateway/android.js

@@ -0,0 +1,43 @@
+"use strict";
+
+const {isIP} = require("net");
+const execa = require("execa");
+
+const args = {
+  v4: ["-4", "r"],
+  v6: ["-6", "r"],
+};
+
+const parse = stdout => {
+  let result;
+
+  (stdout || "").trim().split("\n").some(line => {
+    const [_, gateway, iface] = /default via (.+?) dev (.+?)( |$)/.exec(line) || [];
+    if (gateway && isIP(gateway)) {
+      result = {gateway, interface: (iface ? iface : null)};
+      return true;
+    }
+  });
+
+  if (!result) {
+    throw new Error("Unable to determine default gateway");
+  }
+
+  return result;
+};
+
+const promise = async family => {
+  const {stdout} = await execa("ip", args[family]);
+  return parse(stdout, family);
+};
+
+const sync = family => {
+  const {stdout} = execa.sync("ip", args[family]);
+  return parse(stdout);
+};
+
+module.exports.v4 = () => promise("v4");
+module.exports.v6 = () => promise("v6");
+
+module.exports.v4.sync = () => sync("v4");
+module.exports.v6.sync = () => sync("v6");