📦 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/full/weak-set/add-all.js

@@ -0,0 +1,6 @@
+'use strict';
+require('../../modules/es.weak-set');
+require('../../modules/esnext.weak-set.add-all');
+var entryUnbind = require('../../internals/entry-unbind');
+
+module.exports = entryUnbind('WeakSet', 'addAll');

frontend/node_modules/core-js/full/weak-set/delete-all.js

@@ -0,0 +1,6 @@
+'use strict';
+require('../../modules/es.weak-set');
+require('../../modules/esnext.weak-set.delete-all');
+var entryUnbind = require('../../internals/entry-unbind');
+
+module.exports = entryUnbind('WeakSet', 'deleteAll');

frontend/node_modules/core-js/full/weak-set/from.js

@@ -0,0 +1,12 @@
+'use strict';
+require('../../modules/es.object.to-string');
+require('../../modules/es.array.iterator');
+require('../../modules/es.string.iterator');
+require('../../modules/es.weak-set');
+require('../../modules/esnext.weak-set.from');
+require('../../modules/esnext.weak-set.add-all');
+require('../../modules/esnext.weak-set.delete-all');
+require('../../modules/web.dom-collections.iterator');
+var path = require('../../internals/path');
+
+module.exports = path.WeakSet.from;

frontend/node_modules/core-js/full/weak-set/index.js

@@ -0,0 +1,9 @@
+'use strict';
+var parent = require('../../actual/weak-set');
+require('../../modules/es.string.iterator');
+require('../../modules/esnext.weak-set.add-all');
+require('../../modules/esnext.weak-set.delete-all');
+require('../../modules/esnext.weak-set.from');
+require('../../modules/esnext.weak-set.of');
+
+module.exports = parent;

frontend/node_modules/core-js/full/weak-set/of.js

@@ -0,0 +1,10 @@
+'use strict';
+require('../../modules/es.object.to-string');
+require('../../modules/es.array.iterator');
+require('../../modules/es.weak-set');
+require('../../modules/esnext.weak-set.of');
+require('../../modules/esnext.weak-set.add-all');
+require('../../modules/esnext.weak-set.delete-all');
+var path = require('../../internals/path');
+
+module.exports = path.WeakSet.of;