Add custom sorting

### Describe the feature

Repositories are now sorted by number of PRs, we can add a dropdown to add some more custom sorting

### It should be implemented because

_No response_

### Did you think about any alternative?

_No response_

### Additional context

_No response_

### Extra fields

- [X] I have used the search function to check if an issue already exists
- [ ] I'd like to work on this issue

该 issue 没有指明任何文件、测试、排序选项或 UI 入口。先定位现有的 repository 排序实现及其 frontend 视图，然后定义受支持的下拉选项，并验证每个选项都会改变显示的 repository 顺序。

由索引模型根据 Issue 内容生成。