📦 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-nesting/README.md

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+# PostCSS Nesting [<img src="https://postcss.github.io/postcss/logo.svg" alt="PostCSS" width="90" height="90" align="right">][postcss]
+
+[![NPM Version][npm-img]][npm-url]
+[![CSS Standard Status][css-img]][css-url]
+[<img alt="Discord" src="https://shields.io/badge/Discord-5865F2?logo=discord&logoColor=white">][discord]
+
+[PostCSS Nesting] lets you nest style rules inside each other, following the
+[CSS Nesting] specification. If you want nested rules the same way [Sass] works
+you might want to use [PostCSS Nested] instead.
+
+```pcss
+a, b {
+	color: red;
+
+	/* "&" comes first */
+	& c, & d {
+		color: white;
+	}
+
+	/* "&" comes later, requiring "@nest" */
+	@nest e & {
+		color: yellow;
+	}
+}
+
+/* becomes */
+
+a, b {
+	color: red;
+}
+
+a c, a d, b c, b d {
+	color: white;
+}
+
+e a, e b {
+	color: yellow;
+}
+```
+
+## Usage
+
+Add [PostCSS Nesting] to your project:
+
+```bash
+npm install postcss-nesting --save-dev
+```
+
+Use [PostCSS Nesting] as a [PostCSS] plugin:
+
+```js
+import postcss from 'postcss';
+import postcssNesting from 'postcss-nesting';
+
+postcss([
+  postcssNesting(/* pluginOptions */)
+]).process(YOUR_CSS /*, processOptions */);
+```
+
+[PostCSS Nesting] runs in all Node environments, with special instructions for:
+
+| [Node](INSTALL.md#node) | [Webpack](INSTALL.md#webpack) | [Create React App](INSTALL.md#create-react-app) | [Gulp](INSTALL.md#gulp) | [Grunt](INSTALL.md#grunt) |
+| --- | --- | --- | --- | --- |
+
+### Deno
+
+You can also use [PostCSS Nesting] on [Deno]:
+
+```js
+import postcss from "https://deno.land/x/postcss/mod.js";
+import postcssNesting from "https://cdn.jsdelivr.net/npm/postcss-nesting@10/mod.js";
+
+await postcss([postcssNesting]).process(YOUR_CSS /*, processOptions */);
+```
+
+## Options
+
+### noIsPseudoSelector
+
+#### Specificity
+
+Before :
+
+```css
+#alpha,
+.beta {
+	&:hover {
+		order: 1;
+	}
+}
+```
+
+After **without** the option :
+
+```js
+postcssNesting()
+```
+
+```css
+:is(#alpha,.beta):hover {
+	order: 1;
+}
+```
+
+_`.beta:hover` has specificity as if `.beta` where an id selector, matching the specification._
+
+[specificity: 1, 1, 0](https://polypane.app/css-specificity-calculator/#selector=%3Ais(%23alpha%2C.beta)%3Ahover)
+
+After **with** the option :
+
+```js
+postcssNesting({
+	noIsPseudoSelector: true
+})
+```
+
+```css
+#alpha:hover, .beta:hover {
+	order: 1;
+}
+```
+
+_`.beta:hover` has specificity as if `.beta` where a class selector, conflicting with the specification._
+
+[specificity: 0, 2, 0](https://polypane.app/css-specificity-calculator/#selector=.beta%3Ahover)
+
+
+#### Complex selectors
+
+Before :
+
+```css
+.alpha > .beta {
+	& + & {
+		order: 2;
+	}
+}
+```
+
+After **without** the option :
+
+```js
+postcssNesting()
+```
+
+```css
+:is(.alpha > .beta) + :is(.alpha > .beta) {
+	order: 2;
+}
+```
+
+After **with** the option :
+
+```js
+postcssNesting({
+	noIsPseudoSelector: true
+})
+```
+
+```css
+.alpha > .beta + .alpha > .beta {
+	order: 2;
+}
+```
+
+_this is a different selector than expected as `.beta + .alpha` matches `.beta` followed by `.alpha`._<br>
+_avoid these cases when you disable `:is()`_<br>
+_writing the selector without nesting is advised here_
+
+```css
+/* without nesting */
+.alpha > .beta + .beta {
+	order: 2;
+}
+```
+
+### ⚠️ Spec disclaimer
+
+The [CSS Nesting Module] spec states on nesting that "Declarations occurring after a nested rule are invalid and ignored.".
+While we think it makes sense on browsers, enforcing this at the plugin level introduces several constraints that would
+interfere with PostCSS' plugin nature such as with `@mixin`
+
+[css-img]: https://cssdb.org/images/badges/nesting-rules.svg
+[css-url]: https://cssdb.org/#nesting-rules
+[discord]: https://discord.gg/bUadyRwkJS
+[npm-img]: https://img.shields.io/npm/v/postcss-nesting.svg
+[npm-url]: https://www.npmjs.com/package/postcss-nesting
+
+[CSS Nesting]: https://drafts.csswg.org/css-nesting-1/
+[PostCSS]: https://github.com/postcss/postcss
+[PostCSS Nesting]: https://github.com/csstools/postcss-plugins/tree/main/plugins/postcss-nesting
+[Deno]: https://deno.land/x/postcss_nesting
+[PostCSS Nested]: https://github.com/postcss/postcss-nested
+[Sass]: https://sass-lang.com/
+[CSS Nesting Module]: https://www.w3.org/TR/css-nesting-1/