📦 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/syntax/node/AtrulePrelude.js

@@ -0,0 +1,51 @@
+var TYPE = require('../../tokenizer').TYPE;
+
+var SEMICOLON = TYPE.Semicolon;
+var LEFTCURLYBRACKET = TYPE.LeftCurlyBracket;
+
+module.exports = {
+    name: 'AtrulePrelude',
+    structure: {
+        children: [[]]
+    },
+    parse: function(name) {
+        var children = null;
+
+        if (name !== null) {
+            name = name.toLowerCase();
+        }
+
+        this.scanner.skipSC();
+
+        if (this.atrule.hasOwnProperty(name) &&
+            typeof this.atrule[name].prelude === 'function') {
+            // custom consumer
+            children = this.atrule[name].prelude.call(this);
+        } else {
+            // default consumer
+            children = this.readSequence(this.scope.AtrulePrelude);
+        }
+
+        this.scanner.skipSC();
+
+        if (this.scanner.eof !== true &&
+            this.scanner.tokenType !== LEFTCURLYBRACKET &&
+            this.scanner.tokenType !== SEMICOLON) {
+            this.error('Semicolon or block is expected');
+        }
+
+        if (children === null) {
+            children = this.createList();
+        }
+
+        return {
+            type: 'AtrulePrelude',
+            loc: this.getLocationFromList(children),
+            children: children
+        };
+    },
+    generate: function(node) {
+        this.children(node);
+    },
+    walkContext: 'atrulePrelude'
+};