Engine: deleteAddressNoteMessage drops the row from the store
and enqueues an address_note_message_remove op. Collab apply
mirrors the existing edit path. AddressNoteChange grows a
messageRemoved case.
GTK: right-click on a user message offers Edit and Delete.
Edit swaps the body label for an inline editor; Save commits
via editUserMessage. Delete confirms first, then removes the
row from the popover and via the engine.