mpvpaper kept running over the new theme's static wallpaper because
nothing was watching the omarchy-theme symlink. Add a small sibling
script that polls ~/.config/omarchy/current/background and, when the
target changes, runs `motion-wallpaper-toggle stop` — which tears down
mpvpaper and respawns swaybg with the new symlink target so the new
theme's bg becomes visible.
- motion-wallpaper-theme-watcher: polls readlink every 2s, exits silently
on non-Omarchy systems where the symlink doesn't exist.
- toggle: spawn alongside the auto-pause watcher; kill alongside it.
- wallpaper.sh: install the new binary.
