📦 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/css-tree/lib/convertor/create.js

@@ -0,0 +1,28 @@
+var List = require('../common/List');
+
+module.exports = function createConvertors(walk) {
+    return {
+        fromPlainObject: function(ast) {
+            walk(ast, {
+                enter: function(node) {
+                    if (node.children && node.children instanceof List === false) {
+                        node.children = new List().fromArray(node.children);
+                    }
+                }
+            });
+
+            return ast;
+        },
+        toPlainObject: function(ast) {
+            walk(ast, {
+                leave: function(node) {
+                    if (node.children && node.children instanceof List) {
+                        node.children = node.children.toArray();
+                    }
+                }
+            });
+
+            return ast;
+        }
+    };
+};

frontend/node_modules/css-tree/lib/convertor/index.js

@@ -0,0 +1,3 @@
+var createConvertor = require('./create');
+
+module.exports = createConvertor(require('../walker'));