feat(theme): add toasts to the public site
A stack at the top centre shows the result of an action wherever it was taken: success, info, warning and error toasts that close when their bar runs out and pause under the pointer or focus. The server asks for one with the toast component, which stays an alert without the script; a script calls toast() or announce(). The style guide shows them.
News comments use them: a deleted comment, and a refused deletion, are toasts; a comment just added lights up and its message is only read out to a screen reader.