Saving the notifications form with a blank email box while events are enabled
wiped the customer's hub-side alert address (SyncPreferences pushed empty) —
the 2026-07-15 demo incident. settingsNotificationsHandler now refuses that
save before SetNotificationPrefs + hub sync, re-renders a Hungarian error, and
repaints the submitted checkboxes. Empty email + zero events (clear-all) still
proceeds. Tests + red-proof (remove guard -> stored email wiped to empty).
