feat(notifications): add unified multi-channel notification system

Introduces a fully featured notification system with three delivery
channels (in-app, email, webhook), normalized per-user/per-event/
per-channel preferences, admin-scoped notifications, scheduled trip
reminders and version update alerts.

- New notificationService.send() as the single orchestration entry point
- In-app notifications with simple/boolean/navigate types and WebSocket push
- Per-user preference matrix with normalized notification_channel_preferences table
- Admin notification preferences stored globally in app_settings
- Migration 69 normalizes legacy notification_preferences table
- Scheduler hooks for daily trip reminders and version checks
- DevNotificationsPanel for testing in dev mode
- All new tests passing, covering dispatch, preferences, migration, boolean
  responses, resilience, and full API integration (NSVC, NPREF, INOTIF,
  MIGR, VNOTIF, NROUTE series)
 - Previous tests passing

client/src/App.tsx

@@ -43,7 +43,8 @@ function ProtectedRoute({ children, adminRequired = false }: ProtectedRouteProps
   }
 
   if (!isAuthenticated) {
-    return <Navigate to="/login" replace />
+    const redirectParam = encodeURIComponent(location.pathname + location.search)
+    return <Navigate to={`/login?redirect=${redirectParam}`} replace />
   }
 
   if (