feat: add in-app notification system with real-time delivery

Introduces a full in-app notification system with three types (simple,
boolean with server-side callbacks, navigate), three scopes (user, trip,
admin), fan-out persistence per recipient, and real-time push via
WebSocket. Includes a notification bell in the navbar, dropdown, dedicated
/notifications page, and a dev-only admin tab for testing all notification
variants.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/services/authService.ts

@@ -218,6 +218,7 @@ export function getAppConfig(authenticatedUser: { id: number } | null) {
     notification_channel: notifChannel,
     trip_reminders_enabled: tripRemindersEnabled,
     permissions: authenticatedUser ? getAllPermissions() : undefined,
+    dev_mode: process.env.NODE_ENV === 'development',
   };
 }