Handle user_ticket_login
Legacy Bambu network plugins completed embedded login with
user_login, which the WebView dialog already handled.
Newer Bambu login flows can complete with user_ticket_login and
return only a short-lived ticket. The external browser path already
worked because the local HTTP callback server exchanges that ticket
for tokens, fetches the user profile, and passes the resulting session
payload to change_user.
Mirror that ticket exchange path for embedded WebView login so the
dialog can handle user_ticket_login instead of silently ignoring it
after verification-code submission.
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