Sites behind Basic/Digest auth (silentmode.st/guardian/admin) rendered the
server's 401 page because nothing listened for Electron's login event,
which cancels every challenge by default. A modal sign-in prompt now asks
for the credentials and answers the challenge; Cancel leaves the 401 page.
Concurrent challenges for the same host and realm share one prompt while it
is open, and a rejected answer re-prompts instead of replaying the same
credentials until Chromium gives up with ERR_TOO_MANY_RETRIES.
Also: THESEUS_NO_UPDATE_CHECK skips the release check, for throwaway dev
instances — the one-click install chip they show targets the real install.
