Fix demo mode showing real user data after logout

React Query cache was shared between /app (real API) and /demo/app
(mock data) because query keys were identical. After login→logout→demo,
stale real data served from cache.

Two fixes:
1. Clear React Query cache on logout (auth store)
2. Namespace all query keys with basePath prefix so /app and /demo/app
   caches are completely isolated

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

src/lib/hooks/use-auth.ts

@@ -1,13 +1,14 @@
 "use client";
 
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
-import { useDataProvider } from '@/lib/demo/data-provider-context';
+import { useDataProvider, useQueryKeyPrefix } from '@/lib/demo/data-provider-context';
 import { useRouter } from 'next/navigation';
 
 export function useCurrentUser() {
   const { auth } = useDataProvider();
+  const qk = useQueryKeyPrefix();
   return useQuery({
-    queryKey: ['auth', 'user'],
+    queryKey: qk('auth', 'user'),
     queryFn: () => auth.getCurrentUser(),
     retry: false,
     staleTime: 5 * 60 * 1000, // 5 minutes