mirror-local-state-to-a-remote-account

An opt-in switch that mirrors a local flag onto a signed-in remote account — turning it on back-fills everything already flagged, turning it off stops mirroring and deliberately does not undo, and the per-item call is three-valued so "not attempted" is distinguishable from "failed". Covers writing locally first and unconditionally, which of the two paths is allowed to speak to the user, and why the back-fill is…