📦 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/modules/esnext.set.reduce.js

@@ -0,0 +1,28 @@
+'use strict';
+var $ = require('../internals/export');
+var aCallable = require('../internals/a-callable');
+var aSet = require('../internals/a-set');
+var iterate = require('../internals/set-iterate');
+
+var $TypeError = TypeError;
+
+// `Set.prototype.reduce` method
+// https://github.com/tc39/proposal-collection-methods
+$({ target: 'Set', proto: true, real: true, forced: true }, {
+  reduce: function reduce(callbackfn /* , initialValue */) {
+    var set = aSet(this);
+    var noInitial = arguments.length < 2;
+    var accumulator = noInitial ? undefined : arguments[1];
+    aCallable(callbackfn);
+    iterate(set, function (value) {
+      if (noInitial) {
+        noInitial = false;
+        accumulator = value;
+      } else {
+        accumulator = callbackfn(accumulator, value, value, set);
+      }
+    });
+    if (noInitial) throw new $TypeError('Reduce of empty set with no initial value');
+    return accumulator;
+  }
+});