Add modular group verification system with frontend and backend checks

This commit implements a comprehensive, reusable group verification system:

Components:
- GroupCheckButton: Base component for group verification
- 7 specialized buttons: 3 real groups (authentik Admins, grupo-prueba, lvl0), 1 public access test, 2 system verification buttons
- All buttons support both frontend and backend verification modes

Backend:
- New API endpoint /api/auth/check-group for server-side group validation
- Reads Authentik headers and validates group membership

Frontend:
- Enhanced useAuthentik composable with hasGroup() and checkGroupBackend() methods
- Toast notifications for all verification results
- Smooth animations and color-coded visual feedback

UI Improvements:
- Organized layout with cards for different verification types
- Grid layout for group buttons
- Professional styling with hover effects and shadows
- Clear visual distinction between frontend/backend checks

nuxt4/app/components/auth/CheckAuthentikAdminsButton.vue

@@ -0,0 +1,20 @@
+<template>
+  <AuthGroupCheckButton
+    group-name="authentik Admins"
+    label="Authentik Admins"
+    icon="i-heroicons-shield-check"
+    color="red"
+    variant="soft"
+    :verify-backend="verifyBackend"
+  />
+</template>
+
+<script setup lang="ts">
+interface Props {
+  verifyBackend?: boolean
+}
+
+withDefaults(defineProps<Props>(), {
+  verifyBackend: false
+})
+</script>