mt-navbar 不支持 动态 mt-tab-item数据，默认选中效果失效

### Mint UI 2.0
示例代码：
```

{{item.title}}

如果是动态的tab-item数据，则通过设置v-model 中 selected的值，
默认选中效果失效，改成 注释中的静态数据设置 selected=‘1’ 时初始有 默认选中效果。

Reproduce the issue with the dynamic mt-tab-item example and compare it with the commented static items, focusing on the mt-navbar and mt-tab-item component entry points. Verify that setting selected through v-model initially selects the matching dynamic item, then add or update coverage for both dynamic and static data cases.

Written by the indexing model from the issue text.