feat: restructure settings menu + add automation/agents to topbar

TopBar:
- Add Automation and Agenten to user dropdown menu

Settings restructure:
- Stammdaten: Firmendaten, Adressen (CRUD), Konten (CRUD), Sonstige Daten
- Nutzerverwaltung: Benutzer, Rollen, Gruppen as tabs
- System: Menü, Theme, Plugins as tabs
- Reduce nav items from 15+ to 8
- Add end prop to all settings NavLinks

frontend/src/pages/SettingsUserManagement.tsx

@@ -0,0 +1,48 @@
+import React, { useState } from 'react';
+import { useTranslation } from 'react-i18next';
+import { SettingsUsersPage } from './SettingsUsers';
+import { SettingsRolesPage } from './SettingsRoles';
+import { SettingsGroupsPage } from './SettingsGroups';
+
+export function SettingsUserManagementPage() {
+  const { t } = useTranslation();
+  const [activeTab, setActiveTab] = useState('users');
+
+  const tabs = [
+    { key: 'users', label: t('settings.users', 'Benutzer') },
+    { key: 'roles', label: t('settings.roles', 'Rollen') },
+    { key: 'groups', label: t('settings.groups', 'Gruppen') },
+  ];
+
+  return (
+    <div className="space-y-6" data-testid="settings-user-management-page">
+      <h1 className="text-2xl font-bold text-secondary-900">Nutzerverwaltung</h1>
+
+      <div className="border-b border-secondary-200">
+        <nav className="flex gap-4" role="tablist" aria-label="Nutzerverwaltung Tabs">
+          {tabs.map((tab) => (
+            <button
+              key={tab.key}
+              onClick={() => setActiveTab(tab.key)}
+              role="tab"
+              aria-selected={activeTab === tab.key}
+              className={`px-4 py-2 text-sm font-medium border-b-2 transition-colors ${
+                activeTab === tab.key
+                  ? 'border-primary-500 text-primary-700'
+                  : 'border-transparent text-secondary-600 hover:text-secondary-900 hover:border-secondary-300'
+              }`}
+            >
+              {tab.label}
+            </button>
+          ))}
+        </nav>
+      </div>
+
+      <div role="tabpanel">
+        {activeTab === 'users' && <SettingsUsersPage />}
+        {activeTab === 'roles' && <SettingsRolesPage />}
+        {activeTab === 'groups' && <SettingsGroupsPage />}
+      </div>
+    </div>
+  );
+}