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>

client/src/hooks/useInAppNotificationListener.ts

@@ -0,0 +1,20 @@
+import { useEffect } from 'react'
+import { addListener, removeListener } from '../api/websocket'
+import { useNotificationStore } from '../store/notificationStore'
+
+export function useNotificationListener(): void {
+  const handleNew = useNotificationStore(s => s.handleNewNotification)
+  const handleUpdated = useNotificationStore(s => s.handleUpdatedNotification)
+
+  useEffect(() => {
+    const listener = (event: Record<string, unknown>) => {
+      if (event.type === 'notification:new') {
+        handleNew(event.notification as any)
+      } else if (event.type === 'notification:updated') {
+        handleUpdated(event.notification as any)
+      }
+    }
+    addListener(listener)
+    return () => removeListener(listener)
+  }, [handleNew, handleUpdated])
+}