httpgd package not currently on CRAN

[hturner]

The {httpgd} package was archived on 2025-04-23. Potentially it will get back on CRAN at a later date, but in the meantime we should update the Dockerfile to prefer r-universe over CRAN, so that we install the development version of the package.

This also has the advantage of installing from a Linux binary rather than from source, but also means we need to update the Dockerfile to use ubuntu latest (see also #200, #213) as binaries are only built for the latest version of ubuntu.

[hturner]

Fixed by #235.

[hturner]

Unfortunately httpgd is not currently available on r-universe either. I don't see anyone coming up with a good alternative that will work in the browser (https://github.com/REditorSupport/vscode-R/wiki/Plot-viewer still refers to httpgd). So I think we'll have to install from GitHub.

This means updating our Dockerfile to install {remotes} as well as {languageserver} and then install {httpgd} via remotes::install_github("nx10/httpgd"). On the plus side, {remotes} and {languageserver} can be installed directly from CRAN, so we don't have to customise the repos.

[hturner]

Thinking a bit more, it is still worth using the CRAN r-universe, so we can use the linux binaries for faster package installation. We should also install {gdiff} to help with #271 (needs pre-requisites to be installed, check comments on that issue!).

[hturner]

We should also document the additional packages in .Rprofile, after the list of packages installed for the VSCode-R extension, e.g.

# The remotes package is installed to install httpgd from GitHub
# The gdiff package is installed to support visual difference testing

[hturner]

Closed by #274