📦 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/math/acosh.js

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

frontend/node_modules/core-js-pure/actual/math/asinh.js

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

frontend/node_modules/core-js-pure/actual/math/atanh.js

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

frontend/node_modules/core-js-pure/actual/math/cbrt.js

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

frontend/node_modules/core-js-pure/actual/math/clz32.js

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+'use strict';
+var parent = require('../../stable/math/clz32');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/math/cosh.js

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

frontend/node_modules/core-js-pure/actual/math/expm1.js

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+'use strict';
+var parent = require('../../stable/math/expm1');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/math/f16round.js

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+'use strict';
+var parent = require('../../stable/math/f16round');
+require('../../modules/esnext.math.f16round');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/math/fround.js

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

frontend/node_modules/core-js-pure/actual/math/hypot.js

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

frontend/node_modules/core-js-pure/actual/math/imul.js

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

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

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+'use strict';
+var parent = require('../../stable/math');
+require('../../modules/esnext.math.f16round');
+require('../../modules/esnext.math.sum-precise');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/math/log10.js

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+'use strict';
+var parent = require('../../stable/math/log10');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/math/log1p.js

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+'use strict';
+var parent = require('../../stable/math/log1p');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/math/log2.js

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+'use strict';
+var parent = require('../../stable/math/log2');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/math/sign.js

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

frontend/node_modules/core-js-pure/actual/math/sinh.js

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

frontend/node_modules/core-js-pure/actual/math/sum-precise.js

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+'use strict';
+var parent = require('../../stable/math/sum-precise');
+require('../../modules/esnext.math.sum-precise');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/math/tanh.js

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

frontend/node_modules/core-js-pure/actual/math/to-string-tag.js

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+'use strict';
+var parent = require('../../stable/math/to-string-tag');
+
+module.exports = parent;

frontend/node_modules/core-js-pure/actual/math/trunc.js

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