📦 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/properties/every-values-pair.js

@@ -0,0 +1,28 @@
+var Marker = require('../../../tokenizer/marker');
+
+function everyValuesPair(fn, left, right) {
+  var leftSize = left.value.length;
+  var rightSize = right.value.length;
+  var total = Math.max(leftSize, rightSize);
+  var lowerBound = Math.min(leftSize, rightSize) - 1;
+  var leftValue;
+  var rightValue;
+  var position;
+
+  for (position = 0; position < total; position++) {
+    leftValue = left.value[position] && left.value[position][1] || leftValue;
+    rightValue = right.value[position] && right.value[position][1] || rightValue;
+
+    if (leftValue == Marker.COMMA || rightValue == Marker.COMMA) {
+      continue;
+    }
+
+    if (!fn(leftValue, rightValue, position, position <= lowerBound)) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+module.exports = everyValuesPair;