📦 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/function/element.js

@@ -0,0 +1,13 @@
+// https://drafts.csswg.org/css-images-4/#element-notation
+// https://developer.mozilla.org/en-US/docs/Web/CSS/element
+module.exports = function() {
+    this.scanner.skipSC();
+
+    var children = this.createSingleNodeList(
+        this.IdSelector()
+    );
+
+    this.scanner.skipSC();
+
+    return children;
+};

frontend/node_modules/css-tree/lib/syntax/function/expression.js

@@ -0,0 +1,7 @@
+// legacy IE function
+// expression( <any-value> )
+module.exports = function() {
+    return this.createSingleNodeList(
+        this.Raw(this.scanner.tokenIndex, null, false)
+    );
+};

frontend/node_modules/css-tree/lib/syntax/function/var.js

@@ -0,0 +1,26 @@
+var TYPE = require('../../tokenizer').TYPE;
+var rawMode = require('../node/Raw').mode;
+
+var COMMA = TYPE.Comma;
+
+// var( <ident> , <value>? )
+module.exports = function() {
+    var children = this.createList();
+
+    this.scanner.skipSC();
+
+    // NOTE: Don't check more than a first argument is an ident, rest checks are for lexer
+    children.push(this.Identifier());
+
+    this.scanner.skipSC();
+
+    if (this.scanner.tokenType === COMMA) {
+        children.push(this.Operator());
+        children.push(this.parseCustomProperty
+            ? this.Value(null)
+            : this.Raw(this.scanner.tokenIndex, rawMode.exclamationMarkOrSemicolon, false)
+        );
+    }
+
+    return children;
+};