📦 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/clean-css/lib/optimizer/level-2/specificities-overlap.js

@@ -0,0 +1,36 @@
+var specificity = require('./specificity');
+
+function specificitiesOverlap(selector1, selector2, cache) {
+  var specificity1;
+  var specificity2;
+  var i, l;
+  var j, m;
+
+  for (i = 0, l = selector1.length; i < l; i++) {
+    specificity1 = findSpecificity(selector1[i][1], cache);
+
+    for (j = 0, m = selector2.length; j < m; j++) {
+      specificity2 = findSpecificity(selector2[j][1], cache);
+
+      if (specificity1[0] === specificity2[0]
+        && specificity1[1] === specificity2[1]
+        && specificity1[2] === specificity2[2]) {
+        return true;
+      }
+    }
+  }
+
+  return false;
+}
+
+function findSpecificity(selector, cache) {
+  var value;
+
+  if (!(selector in cache)) {
+    cache[selector] = value = specificity(selector);
+  }
+
+  return value || cache[selector];
+}
+
+module.exports = specificitiesOverlap;