📦 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/number/constructor.js

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+'use strict';
+var parent = require('../../stable/number/constructor');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/epsilon.js

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+'use strict';
+var parent = require('../../stable/number/epsilon');
+
+module.exports = parent;

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

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+'use strict';
+var parent = require('../../stable/number');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/is-finite.js

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+'use strict';
+var parent = require('../../stable/number/is-finite');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/is-integer.js

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+'use strict';
+var parent = require('../../stable/number/is-integer');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/is-nan.js

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+'use strict';
+var parent = require('../../stable/number/is-nan');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/is-safe-integer.js

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+'use strict';
+var parent = require('../../stable/number/is-safe-integer');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/max-safe-integer.js

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+'use strict';
+var parent = require('../../stable/number/max-safe-integer');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/min-safe-integer.js

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+'use strict';
+var parent = require('../../stable/number/min-safe-integer');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/parse-float.js

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+'use strict';
+var parent = require('../../stable/number/parse-float');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/parse-int.js

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+'use strict';
+var parent = require('../../stable/number/parse-int');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/to-exponential.js

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+'use strict';
+var parent = require('../../stable/number/to-exponential');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/to-fixed.js

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+'use strict';
+var parent = require('../../stable/number/to-fixed');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/to-precision.js

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+'use strict';
+var parent = require('../../stable/number/to-precision');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/virtual/index.js

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+'use strict';
+var parent = require('../../../stable/number/virtual');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/virtual/to-exponential.js

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+'use strict';
+var parent = require('../../../stable/number/virtual/to-exponential');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/virtual/to-fixed.js

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+'use strict';
+var parent = require('../../../stable/number/virtual/to-fixed');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/number/virtual/to-precision.js

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+'use strict';
+var parent = require('../../../stable/number/virtual/to-precision');
+
+module.exports = parent;