Keyed draft stash (Map + localStorage mirror) behind the live composer:
switching threads stashes the departing draft and restores the entering
one; empty threads show an empty box. Session lifecycle never clears
composer state — the scope swap is the only coupling.
Co-authored-by: mollusk <roger@roger.local>
Co-authored-by: Teknium <127238744+teknium1@users.noreply.github.com>