Feature request: Warn for, or block, local-only posts that mention remote profiles

[pronoiac]

Pitch

Currently, if you:

• @ mention someone on another instance AND
• select Local-only THEN
• when you post: it quietly drops the recipients. Like mentioning @obviouslyfakeuser here.

I suggest:

• when you try to post, you get an error, like, say: "it looks like you're trying to @ mention someone on a remote instance, but the selected local-only post settings would prevent them seeing this post"

(I'm wincing that it sounds like Clippy, but that's the first voice that it came out in.)

Motivation

Accidentally, quietly dropping recipients can be an unpleasant surprise, that a friend has possibly been getting wrong for months. A block with warning could be less surprising and better match their expectations.