explicitly prohibit editing a note's visibility
changing the visibility of a note doesn't really work with federation: if you narrow the visibility, some instances that received the Create, may not see the Update at all; even those who receive both the Create and the Update, will probably not hide the updated contents and show the old ones to the people covered by the old visibility and not the new one… if you widen visibility, more instances will get the Update than got the Create, so they'll ignore it…
and other fedi software straight up prohibits changing it, e.g. IceShrimp
so…
- disable the visibility drop-down in the posting dialog when editing
- throw an error in the backend if a request tries to edit visibility