📦 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/axios/lib/platform/node/classes/FormData.js

@@ -0,0 +1,3 @@
+import FormData from 'form-data';
+
+export default FormData;

frontend/node_modules/axios/lib/platform/node/classes/URLSearchParams.js

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+'use strict';
+
+import url from 'url';
+export default url.URLSearchParams;

frontend/node_modules/axios/lib/platform/node/index.js

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+import crypto from 'crypto';
+import URLSearchParams from './classes/URLSearchParams.js';
+import FormData from './classes/FormData.js';
+
+const ALPHA = 'abcdefghijklmnopqrstuvwxyz';
+
+const DIGIT = '0123456789';
+
+const ALPHABET = {
+  DIGIT,
+  ALPHA,
+  ALPHA_DIGIT: ALPHA + ALPHA.toUpperCase() + DIGIT,
+};
+
+const generateString = (size = 16, alphabet = ALPHABET.ALPHA_DIGIT) => {
+  let str = '';
+  const { length } = alphabet;
+  const randomValues = new Uint32Array(size);
+  crypto.randomFillSync(randomValues);
+  for (let i = 0; i < size; i++) {
+    str += alphabet[randomValues[i] % length];
+  }
+
+  return str;
+};
+
+export default {
+  isNode: true,
+  classes: {
+    URLSearchParams,
+    FormData,
+    Blob: (typeof Blob !== 'undefined' && Blob) || null,
+  },
+  ALPHABET,
+  generateString,
+  protocols: ['http', 'https', 'file', 'data'],
+};