📦 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/actual/set/difference.js

@@ -0,0 +1,5 @@
+'use strict';
+var parent = require('../../stable/set/difference');
+require('../../modules/esnext.set.difference.v2');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/set/index.js

@@ -0,0 +1,11 @@
+'use strict';
+var parent = require('../../stable/set');
+require('../../modules/esnext.set.difference.v2');
+require('../../modules/esnext.set.intersection.v2');
+require('../../modules/esnext.set.is-disjoint-from.v2');
+require('../../modules/esnext.set.is-subset-of.v2');
+require('../../modules/esnext.set.is-superset-of.v2');
+require('../../modules/esnext.set.symmetric-difference.v2');
+require('../../modules/esnext.set.union.v2');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/set/intersection.js

@@ -0,0 +1,5 @@
+'use strict';
+var parent = require('../../stable/set/intersection');
+require('../../modules/esnext.set.intersection.v2');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/set/is-disjoint-from.js

@@ -0,0 +1,5 @@
+'use strict';
+var parent = require('../../stable/set/is-disjoint-from');
+require('../../modules/esnext.set.is-disjoint-from.v2');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/set/is-subset-of.js

@@ -0,0 +1,5 @@
+'use strict';
+var parent = require('../../stable/set/is-subset-of');
+require('../../modules/esnext.set.is-subset-of.v2');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/set/is-superset-of.js

@@ -0,0 +1,5 @@
+'use strict';
+var parent = require('../../stable/set/is-superset-of');
+require('../../modules/esnext.set.is-superset-of.v2');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/set/symmetric-difference.js

@@ -0,0 +1,5 @@
+'use strict';
+var parent = require('../../stable/set/symmetric-difference');
+require('../../modules/esnext.set.symmetric-difference.v2');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/set/union.js

@@ -0,0 +1,5 @@
+'use strict';
+var parent = require('../../stable/set/union');
+require('../../modules/esnext.set.union.v2');
+
+module.exports = parent;