Preview a manual publisher dispatch thread

Renders the subject/body/attachment plan of the FIRST message a manual send would produce for the selected articles, without downloading attachment content. For a team on full dispatch the selection goes out as ONE thread with a message per article and message_count reports how many that is; omitted_attachment_count covers the whole thread and counts files exceeding a single message's attachment budget. For any other team (summary: true) the whole selection renders as a single summary digest: message_count is 1, attachments is always empty, and omitted_attachment_count is always 0 because attachments are never computed for a digest.