Trivial improvement for web identity UI

When adding a web identity via DNS it would be nice to display the domain it's being created for in the popup dialog that displays the proof record to add.

I have the attention span of a gnat, and while adding proofs for several domains in a row I managed to forget which one I was up to and add the proof to the wrong domain.

Start by locating the web-identity DNS proof popup in the client UI and tracing where its proof-record text is assembled. Confirm the domain being added is available at that point; done means the popup identifies the target domain while preserving the existing proof instructions.

Written by the indexing model from the issue text.