📦 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/core-js-pure/modules/esnext.string.at.js

@@ -0,0 +1,19 @@
+'use strict';
+// TODO: Remove from `core-js@4`
+var $ = require('../internals/export');
+var charAt = require('../internals/string-multibyte').charAt;
+var requireObjectCoercible = require('../internals/require-object-coercible');
+var toIntegerOrInfinity = require('../internals/to-integer-or-infinity');
+var toString = require('../internals/to-string');
+
+// `String.prototype.at` method
+// https://github.com/mathiasbynens/String.prototype.at
+$({ target: 'String', proto: true, forced: true }, {
+  at: function at(index) {
+    var S = toString(requireObjectCoercible(this));
+    var len = S.length;
+    var relativeIndex = toIntegerOrInfinity(index);
+    var k = relativeIndex >= 0 ? relativeIndex : len + relativeIndex;
+    return (k < 0 || k >= len) ? undefined : charAt(S, k);
+  }
+});