All opt-in; nothing changes until someone picks a setting.
- Keep official mode on (Settings > Chat, saved on the account as
users.official_mode, every device follows through set-official-mode /
official-mode-changed). The megaphone then rests armed, so staff don't
re-arm it for every message; pressing it sends just the next message
plain. The client still sends highlight: true per message, so the wire
format and the server's checks are unchanged.
- Official messages look (Settings > Chat, per device): Badge (the default,
as before), Colored text (the text and a lighter wash in the role's color,
a thinner bar, no badge; text mixed with the theme's own text color, more
of it on light themes), or Off.
- With Colored text, consecutive official messages from one person group
into one block. Badge keeps every official message's own header.
- session-info carries features: ['highlight_messages', 'official_mode'] and
officialMode, so apps can tell what a server supports.
