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

(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.

Start by locating the interactive auth dialog and its password-authentication entry point in the TypeScript client. Confirm where the username and password prompt are rendered, then verify that password authentication shows the username above the prompt without changing other authentication modes.

Written by the indexing model from the issue text.