feat(bookstack) : add BookStack tab to admin page

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

frontend/pages/admin.vue

@@ -28,6 +28,7 @@
             <AdminTagTab v-if="activeTab === 'tags'" />
             <AdminUserTab v-if="activeTab === 'users'" />
             <AdminGiteaTab v-if="activeTab === 'gitea'" />
+            <AdminBookStackTab v-if="activeTab === 'bookstack'" />
         </div>
     </div>
 </template>
@@ -43,6 +44,7 @@ const tabs = [
     { key: 'tags', label: 'Tags' },
     { key: 'users', label: 'Utilisateurs' },
     { key: 'gitea', label: 'Gitea' },
+    { key: 'bookstack', label: 'BookStack' },
 ] as const
 
 type TabKey = typeof tabs[number]['key']