Tweak interactive auth dialog to make it clearer what password we're asking for

[duxovni]

(Splitting this off from #19279 , which covered several independent UI bugs found while reviewing matrix-org/matrix-react-sdk#6811)

In the interactive auth dialog that shows up, among other places, when resetting cross-signing keys: If we're using password authentication, add the username above the password prompt, to make it clearer that we're asking for the login password, rather than the recovery passphrase or something else.