📦 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/index.d.ts

@@ -0,0 +1,26 @@
+import type * as PostCSS from 'postcss'
+
+export type PluginOptions = {
+	/** Determines whether multiple, duplicate insertions are allowed */
+	allowDuplicates?: boolean
+	/** Defines an override of the project’s browserslist for this plugin. */
+	browsers?: string
+	/** Defines whether imports from this plugin will always be inserted at the beginning of a CSS file. */
+	forceImport?: boolean
+}
+
+export type Plugin = {
+	(pluginOptions?: PluginOptions): {
+		postcssPlugin: 'postcss-normalize'
+		Once(root: PostCSS.Root): void
+		postcssImport: {
+			load(filename: string, importOptions: any): {}
+			resolve(id: string, basedir: string, importOptions: any): {}
+		}
+	}
+	postcss: true
+}
+
+declare const plugin: Plugin
+
+export default plugin