📦 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/object.fromentries/implementation.js

@@ -0,0 +1,18 @@
+'use strict';
+
+var AddEntriesFromIterable = require('es-abstract/2024/AddEntriesFromIterable');
+var CreateDataPropertyOrThrow = require('es-abstract/2024/CreateDataPropertyOrThrow');
+var RequireObjectCoercible = require('es-object-atoms/RequireObjectCoercible');
+var ToPropertyKey = require('es-abstract/2024/ToPropertyKey');
+
+var adder = function addDataProperty(key, value) {
+	var O = this; // eslint-disable-line no-invalid-this
+	var propertyKey = ToPropertyKey(key);
+	CreateDataPropertyOrThrow(O, propertyKey, value);
+};
+
+module.exports = function fromEntries(iterable) {
+	RequireObjectCoercible(iterable);
+
+	return AddEntriesFromIterable({}, iterable, adder);
+};