📦 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/postcss-normalize-whitespace/LICENSE-MIT

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+Copyright (c) Ben Briggs <beneb.info@gmail.com> (http://beneb.info)
+
+Permission is hereby granted, free of charge, to any person
+obtaining a copy of this software and associated documentation
+files (the "Software"), to deal in the Software without
+restriction, including without limitation the rights to use,
+copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the
+Software is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+OTHER DEALINGS IN THE SOFTWARE.

frontend/node_modules/postcss-normalize-whitespace/README.md

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+# [postcss][postcss]-normalize-whitespace
+
+> Normalize whitespace with PostCSS.
+
+## Install
+
+With [npm](https://npmjs.org/package/postcss-normalize-whitespace) do:
+
+```
+npm install postcss-normalize-whitespace --save
+```
+
+## Example
+
+### Input
+
+```css
+h1{
+    width: calc(10px - ( 100px / var(--test) )) 
+}
+```
+
+### Output
+
+```css
+h1{
+    width: calc(10px - 100px / var(--test))
+}
+``` 
+
+## Usage
+
+See the [PostCSS documentation](https://github.com/postcss/postcss#usage) for
+examples for your environment.
+
+## Contributors
+
+See [CONTRIBUTORS.md](https://github.com/cssnano/cssnano/blob/master/CONTRIBUTORS.md).
+
+## License
+
+MIT © [Ben Briggs](http://beneb.info)
+
+[postcss]: https://github.com/postcss/postcss

frontend/node_modules/postcss-normalize-whitespace/package.json

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+{
+  "name": "postcss-normalize-whitespace",
+  "version": "5.1.1",
+  "description": "Trim whitespace inside and around CSS rules & declarations.",
+  "main": "src/index.js",
+  "types": "types/index.d.ts",
+  "files": [
+    "src",
+    "LICENSE-MIT",
+    "types"
+  ],
+  "keywords": [
+    "css",
+    "postcss",
+    "postcss-plugin"
+  ],
+  "license": "MIT",
+  "homepage": "https://github.com/cssnano/cssnano",
+  "author": {
+    "name": "Ben Briggs",
+    "email": "beneb.info@gmail.com",
+    "url": "http://beneb.info"
+  },
+  "repository": "cssnano/cssnano",
+  "dependencies": {
+    "postcss-value-parser": "^4.2.0"
+  },
+  "bugs": {
+    "url": "https://github.com/cssnano/cssnano/issues"
+  },
+  "engines": {
+    "node": "^10 || ^12 || >=14.0"
+  },
+  "devDependencies": {
+    "postcss": "^8.2.15"
+  },
+  "peerDependencies": {
+    "postcss": "^8.2.15"
+  },
+  "readme": "# [postcss][postcss]-normalize-whitespace\n\n> Normalize whitespace with PostCSS.\n\n## Install\n\nWith [npm](https://npmjs.org/package/postcss-normalize-whitespace) do:\n\n```\nnpm install postcss-normalize-whitespace --save\n```\n\n## Example\n\n### Input\n\n```css\nh1{\n    width: calc(10px - ( 100px / var(--test) )) \n}\n```\n\n### Output\n\n```css\nh1{\n    width: calc(10px - 100px / var(--test))\n}\n``` \n\n## Usage\n\nSee the [PostCSS documentation](https://github.com/postcss/postcss#usage) for\nexamples for your environment.\n\n## Contributors\n\nSee [CONTRIBUTORS.md](https://github.com/cssnano/cssnano/blob/master/CONTRIBUTORS.md).\n\n## License\n\nMIT © [Ben Briggs](http://beneb.info)\n\n[postcss]: https://github.com/postcss/postcss\n"
+}

frontend/node_modules/postcss-normalize-whitespace/src/index.js

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+'use strict';
+const valueParser = require('postcss-value-parser');
+
+const atrule = 'atrule';
+const decl = 'decl';
+const rule = 'rule';
+const variableFunctions = new Set(['var', 'env', 'constant']);
+
+/**
+ * @param {valueParser.Node} node
+ * @return {void}
+ */
+function reduceCalcWhitespaces(node) {
+  if (node.type === 'space') {
+    node.value = ' ';
+  } else if (node.type === 'function') {
+    if (!variableFunctions.has(node.value.toLowerCase())) {
+      node.before = node.after = '';
+    }
+  }
+}
+/**
+ * @param {valueParser.Node} node
+ * @return {void | false}
+ */
+function reduceWhitespaces(node) {
+  if (node.type === 'space') {
+    node.value = ' ';
+  } else if (node.type === 'div') {
+    node.before = node.after = '';
+  } else if (node.type === 'function') {
+    if (!variableFunctions.has(node.value.toLowerCase())) {
+      node.before = node.after = '';
+    }
+    if (node.value.toLowerCase() === 'calc') {
+      valueParser.walk(node.nodes, reduceCalcWhitespaces);
+      return false;
+    }
+  }
+}
+
+/**
+ * @type {import('postcss').PluginCreator<void>}
+ * @return {import('postcss').Plugin}
+ */
+function pluginCreator() {
+  return {
+    postcssPlugin: 'postcss-normalize-whitespace',
+
+    OnceExit(css) {
+      const cache = new Map();
+
+      css.walk((node) => {
+        const { type } = node;
+
+        if ([decl, rule, atrule].includes(type) && node.raws.before) {
+          node.raws.before = node.raws.before.replace(/\s/g, '');
+        }
+
+        if (type === decl) {
+          // Ensure that !important values do not have any excess whitespace
+          if (node.important) {
+            node.raws.important = '!important';
+          }
+
+          // Remove whitespaces around ie 9 hack
+          node.value = node.value.replace(/\s*(\\9)\s*/, '$1');
+          const value = node.value;
+
+          if (cache.has(value)) {
+            node.value = cache.get(value);
+          } else {
+            const parsed = valueParser(node.value);
+            const result = parsed.walk(reduceWhitespaces).toString();
+
+            // Trim whitespace inside functions & dividers
+            node.value = result;
+            cache.set(value, result);
+          }
+
+          if (node.prop.startsWith('--') && node.value === '') {
+            node.value = ' ';
+          }
+          // Remove extra semicolons and whitespace before the declaration
+          if (node.raws.before) {
+            const prev = node.prev();
+
+            if (prev && prev.type !== rule) {
+              node.raws.before = node.raws.before.replace(/;/g, '');
+            }
+          }
+
+          node.raws.between = ':';
+          node.raws.semicolon = false;
+        } else if (type === rule || type === atrule) {
+          node.raws.between = node.raws.after = '';
+          node.raws.semicolon = false;
+        }
+      });
+
+      // Remove final newline
+      css.raws.after = '';
+    },
+  };
+}
+
+pluginCreator.postcss = true;
+module.exports = pluginCreator;

frontend/node_modules/postcss-normalize-whitespace/types/index.d.ts

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+export = pluginCreator;
+/**
+ * @type {import('postcss').PluginCreator<void>}
+ * @return {import('postcss').Plugin}
+ */
+declare function pluginCreator(): import('postcss').Plugin;
+declare namespace pluginCreator {
+    const postcss: true;
+}