📦 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/core-js-compat/external.json

@@ -0,0 +1,21 @@
+{
+  "modules": {
+    "android": "61",
+    "bun": "0.1.1",
+    "chrome": "61",
+    "chrome-android": "61",
+    "deno": "1.0",
+    "edge": "16",
+    "firefox": "60",
+    "firefox-android": "60",
+    "ios": "10.3",
+    "node": "13.2",
+    "oculus": "4.0",
+    "opera": "48",
+    "opera-android": "45",
+    "opera_mobile": "45",
+    "quest": "4.0",
+    "safari": "10.1",
+    "samsung": "8.0"
+  }
+}