📦 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/web/README.md

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+This folder contains entry points for features from [WHATWG / W3C](https://github.com/zloirock/core-js#web-standards) with dependencies.

frontend/node_modules/core-js/web/dom-collections.js

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+'use strict';
+require('../modules/web.dom-collections.for-each');
+require('../modules/web.dom-collections.iterator');
+var path = require('../internals/path');
+
+module.exports = path;

frontend/node_modules/core-js/web/dom-exception.js

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+'use strict';
+require('../modules/es.error.to-string');
+require('../modules/web.dom-exception.constructor');
+require('../modules/web.dom-exception.stack');
+require('../modules/web.dom-exception.to-string-tag');
+var path = require('../internals/path');
+
+module.exports = path.DOMException;

frontend/node_modules/core-js/web/immediate.js

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+'use strict';
+require('../modules/web.immediate');
+var path = require('../internals/path');
+
+module.exports = path;

frontend/node_modules/core-js/web/index.js

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+'use strict';
+require('../modules/web.atob');
+require('../modules/web.btoa');
+require('../modules/web.dom-collections.for-each');
+require('../modules/web.dom-collections.iterator');
+require('../modules/web.dom-exception.constructor');
+require('../modules/web.dom-exception.stack');
+require('../modules/web.dom-exception.to-string-tag');
+require('../modules/web.immediate');
+require('../modules/web.queue-microtask');
+require('../modules/web.self');
+require('../modules/web.structured-clone');
+require('../modules/web.timers');
+require('../modules/web.url');
+require('../modules/web.url.can-parse');
+require('../modules/web.url.parse');
+require('../modules/web.url.to-json');
+require('../modules/web.url-search-params');
+require('../modules/web.url-search-params.delete');
+require('../modules/web.url-search-params.has');
+require('../modules/web.url-search-params.size');
+var path = require('../internals/path');
+
+module.exports = path;

frontend/node_modules/core-js/web/queue-microtask.js

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+'use strict';
+require('../modules/web.queue-microtask');
+var path = require('../internals/path');
+
+module.exports = path.queueMicrotask;

frontend/node_modules/core-js/web/structured-clone.js

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+'use strict';
+require('../modules/es.array.iterator');
+require('../modules/es.object.to-string');
+require('../modules/es.map');
+require('../modules/es.set');
+require('../modules/web.structured-clone');
+var path = require('../internals/path');
+
+module.exports = path.structuredClone;

frontend/node_modules/core-js/web/timers.js

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+'use strict';
+require('../modules/web.timers');
+var path = require('../internals/path');
+
+module.exports = path;

frontend/node_modules/core-js/web/url-search-params.js

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+'use strict';
+require('../modules/web.url-search-params');
+require('../modules/web.url-search-params.delete');
+require('../modules/web.url-search-params.has');
+require('../modules/web.url-search-params.size');
+var path = require('../internals/path');
+
+module.exports = path.URLSearchParams;

frontend/node_modules/core-js/web/url.js

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+'use strict';
+require('./url-search-params');
+require('../modules/web.url');
+require('../modules/web.url.can-parse');
+require('../modules/web.url.parse');
+require('../modules/web.url.to-json');
+var path = require('../internals/path');
+
+module.exports = path.URL;