修复 embed 跨域并持久化圆桌 Step3 结果

- Gateway 默认 CORS 白名单增加 5174，embed 文档补充跨域配置说明
- 圆桌草稿新增 step3 快照字段与迁移，结果绘制 HTML 可随草稿恢复

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

offline-backend-20260512/backend/app/gateway/app.py

@@ -807,6 +807,10 @@ This gateway provides custom endpoints for models, MCP configuration, skills, an
         dev_origins = [
             "http://localhost:5173",
             "http://127.0.0.1:5173",
+            "http://localhost:5174",
+            "http://127.0.0.1:5174",
+            "http://localhost:3000",
+            "http://127.0.0.1:3000",
         ]
         logger.warning(
             "No CORS configuration found; enabling local dev origins only: %s. "