UserPreferences missing notification_events field in frontend type

## Bug Backend SettingsResponse includes notification_events: dict[str, Any] for granular per-event-type notification settings. Frontend UserPreferences type has no such field. ## Impact Users cannot configure which event types trigger notifications. The granular notification settings UI cannot be built. ## Fix Add notification_events: Record<string, unknown> to frontend UserPreferences type. ## Found by API contract audit (2026-03-19)