Report Gmail thread as spam

Report a Gmail thread as spam by its hex threadId (moves it to Spam). Requires an authenticated mail.google.com session. Works from the All Mail list view and is locale-independent. Returns reportedSpam, threadId. Moving the thread to Spam also feeds Gmail's spam classifier for that sender, which undoing does not fully reverse. Pass `account` (authuser=) in a multi-account browser jar, or Gmail picks u/0 nondeterministically. The thread must be on the first page of All Mail (newest-first) — the script does not paginate, so an older thread will not be found. Senders with an unsubscribe link raise an extra "Report spam or unsubscribe?" dialog; the script confirms the spam branch and rejects the unsubscribe one, and throws rather than guessing on an unrecognised dialog shape.