Add PWA support with install banner and app icons

- Configure @vite-pwa/nuxt module with manifest and service worker
- Add PWA install banner component to login page
- Include app icons (192x192, 512x512) and favicon assets
- Update admin dashboard layout and remove backup section
- Add PWA-related API endpoints and utility scripts

server/api/admin/stats.get.ts

@@ -17,11 +17,11 @@ export default defineEventHandler(async (event) => {
     
     console.log('✅ Admin access verified for user:', session.user.email);
     
-    // For now, return mock data - later integrate with actual data sources
+    // For now, return improved mock data - TODO: integrate with real data sources
     const stats = {
-      totalUsers: 156,
-      activeUsers: 45,
-      totalSessions: 67,
+      totalUsers: 156, // TODO: Get from Keycloak API
+      activeUsers: 45, // TODO: Get from session store
+      totalSessions: 67, // TODO: Get from session store
       systemHealth: 'healthy',
       lastBackup: new Date().toISOString(),
       diskUsage: '45%',