fix: 修复 .gitignore 并降级 React 到 18 以对齐设计文档

- 补充 .gitignore：忽略 node_modules/、dist/、bin/、.env、.obsidian/ 等
- 降级 react、react-dom 到 18.3.1，@types/react、@types/react-dom 到 18.x
- 与 docs/02-系统架构.md 和 CLAUDE.md 中的 React 18 技术选型保持一致

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/package.json

@@ -12,15 +12,15 @@
   "dependencies": {
     "@ricky0123/vad-web": "^0.0.30",
     "onnxruntime-web": "^1.26.0",
-    "react": "^19.2.6",
-    "react-dom": "^19.2.6",
+    "react": "^18.3.1",
+    "react-dom": "^18.3.1",
     "uuid": "^14.0.0"
   },
   "devDependencies": {
     "@eslint/js": "^10.0.1",
     "@types/node": "^24.12.3",
-    "@types/react": "^19.2.14",
-    "@types/react-dom": "^19.2.3",
+    "@types/react": "^18.3.31",
+    "@types/react-dom": "^18.3.7",
     "@types/uuid": "^10.0.0",
     "@vitejs/plugin-react": "^6.0.1",
     "eslint": "^10.3.0",