📦 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/definition-syntax/tokenizer.js

@@ -0,0 +1,55 @@
+var SyntaxError = require('./SyntaxError');
+
+var TAB = 9;
+var N = 10;
+var F = 12;
+var R = 13;
+var SPACE = 32;
+
+var Tokenizer = function(str) {
+    this.str = str;
+    this.pos = 0;
+};
+
+Tokenizer.prototype = {
+    charCodeAt: function(pos) {
+        return pos < this.str.length ? this.str.charCodeAt(pos) : 0;
+    },
+    charCode: function() {
+        return this.charCodeAt(this.pos);
+    },
+    nextCharCode: function() {
+        return this.charCodeAt(this.pos + 1);
+    },
+    nextNonWsCode: function(pos) {
+        return this.charCodeAt(this.findWsEnd(pos));
+    },
+    findWsEnd: function(pos) {
+        for (; pos < this.str.length; pos++) {
+            var code = this.str.charCodeAt(pos);
+            if (code !== R && code !== N && code !== F && code !== SPACE && code !== TAB) {
+                break;
+            }
+        }
+
+        return pos;
+    },
+    substringToPos: function(end) {
+        return this.str.substring(this.pos, this.pos = end);
+    },
+    eat: function(code) {
+        if (this.charCode() !== code) {
+            this.error('Expect `' + String.fromCharCode(code) + '`');
+        }
+
+        this.pos++;
+    },
+    peek: function() {
+        return this.pos < this.str.length ? this.str.charAt(this.pos++) : '';
+    },
+    error: function(message) {
+        throw new SyntaxError(message, this.str, this.pos);
+    }
+};
+
+module.exports = Tokenizer;