One element next to the title: a badge when there is nothing to choose, the same
badge as a dropdown trigger when there is.

## Usage

```vue
<DuxtVersion />
```

## API

No props. Reads `versions`, or the resolved source manifest when that is unset.

## Notes

Before, the project's version sat as a badge beside the title while the
documentation's versions lived in a separate dropdown on the far right — two
controls saying different things, and the right-hand one appeared only on sources
that have versions, so the icons beside it shifted as you moved between
repositories.

Switching version keeps the page: the prefix is swapped rather than the reader
being sent to the version's root. Each entry carries its lifecycle, because a
list where three entries look alike says nothing about which of them is still
safe to read.