login: align browser-flow output with the device flow

Make the default loopback flow read like --device: show "Login URL:", pause on "Press Enter to open in browser...", then print "Waiting for sign-in... " with no newline so persistLogin's "Login complete." lands on the same line. waitForEnter now short-circuits under test (interactive.UnderTest) like openBrowser, so forcing interactive mode in tests no longer blocks on a real /dev/tty read.