📦 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/sanitize.css/system-ui.css

@@ -0,0 +1,27 @@
+/**
+ * Add the correct system-ui font-family in Firefox.
+ */
+
+@font-face {
+  font-family: system-ui;
+  src: local(".AppleSystemUIFont"), local("Segoe UI"), local("Ubuntu"), local("Roboto-Regular"), local("HelveticaNeue");
+}
+
+@font-face {
+  font-family: system-ui;
+  font-style: italic;
+  src: local(".AppleSystemUIFont"), local("Segoe UI Italic"), local("Ubuntu-Italic"), local("Roboto-Italic"), local("HelveticaNeue-Italic");
+}
+
+@font-face {
+  font-family: system-ui;
+  font-weight: bold;
+  src: local(".AppleSystemUIFont"), local("Segoe UI Bold"), local("Ubuntu-Bold"), local("Roboto-Bold"), local("HelveticaNeue-Bold");
+}
+
+@font-face {
+  font-family: system-ui;
+  font-style: italic;
+  font-weight: bold;
+  src: local(".AppleSystemUIFont"), local("Segoe UI Bold Italic"), local("Ubuntu-BoldItalic"), local("Roboto-BoldItalic"), local("HelveticaNeue-BoldItalic");
+}