📦 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/jsx-ast-utils/lib/values/expressions/AssignmentExpression.js

@@ -0,0 +1,19 @@
+'use strict';
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+exports.default = extractValueFromAssignmentExpression;
+/**
+ * Extractor function for a AssignmentExpression type value node.
+ * An assignment expression looks like `x = y` or `x += y` in expression position.
+ * This will return the assignment as the value.
+ *
+ * @param - value - AST Value object with type `AssignmentExpression`
+ * @returns - The extracted value converted to correct type.
+ */
+function extractValueFromAssignmentExpression(value) {
+  // eslint-disable-next-line global-require
+  var getValue = require('.').default;
+  return getValue(value.left) + ' ' + value.operator + ' ' + getValue(value.right);
+}