Add `querychat_app()`

It'd be really nice to be able to just pass a dataframe into a function like `querychat_app()` to spin up a quick querychat instance to explore a dataset. The app would return the `client` containing the conversation history at the end of the chat, like `btw::btw_app()` and `ellmer::live_browser()`.

```r
library(querychat)

mtcars |> querychat_app()
```