Running support: tickets, threading & replies — Internal notes

The Internal note tab writes into the same thread and is never emailed to anybody. The To row disappears, and the note is badged Internal note · not sent so that nobody skim-reading a long ticket mistakes an internal remark for something the customer was told. A note is unsendable by construction rather than by convention: nothing in RowFold mails one, so don't send this is a property of the message rather than a rule every future feature has to remember.