The debounced observer was unreliable: drop(1) plus the ViewModel lifecycle
could swallow a change after re-entering the app, so a single range change
sometimes didn't refresh the widget. Refresh directly from the settings
setters instead (always fires on a real change), serialized through a mutex
so a rapid flip-and-flip-back can't run two updateAll calls at once and
strand the widget on the intermediate value.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>