feat(dashboard-frontend): 新增 dashboard 前端目录与构建产物

- 引入 dashboard-frontend/dist 静态资源占位（Vite 产物：assets + favicon + index.html + mockServiceWorker.js）
- .gitignore 排除 dashboard-frontend/node_modules 及日志，避免误入库（~304MB）
- src 暂留占位待后续接入

Co-authored-by: Cursor <cursoragent@cursor.com>

.gitignore

@@ -20,3 +20,8 @@ backend/**/logs/
 backend/**/*.log
 backend/.idea/
 backend/crmebimage/
+
+# Dashboard frontend
+dashboard-frontend/node_modules/
+dashboard-frontend/npm-debug.log*
+dashboard-frontend/**/*.log