fix: Rollenhierarchie domain_admin/superadmin in Frontend

- useAuth: neue Rollen domain_admin/superadmin + hasRole() helper
- Admin-Page: useAuth(domain_admin), isSuperAdmin-Flag
- Tabs LDAP/Security/Mandanten/Module nur für superadmin
- Navbar: Admin-Link für domain_admin + superadmin sichtbar
- User-Anlage: domain_admin-Rolle wählbar, superadmin nur für superadmin

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

src/app/admin/page.tsx

@@ -89,7 +89,8 @@ function formatBytes(bytes: number): string {
 }
 
 export default function AdminPage() {
-  const { user, loading: authLoading } = useAuth("admin");
+  const { user, loading: authLoading } = useAuth("domain_admin");
+  const isSuperAdmin = user?.role === "superadmin";
 
   // Dashboard state
   const [smtpStatus, setSmtpStatus] = useState<SMTPStatus | null>(null);
@@ -620,10 +621,10 @@ export default function AdminPage() {
             <TabsTrigger value="users">Benutzer</TabsTrigger>
             <TabsTrigger value="audit">Audit-Log</TabsTrigger>
             <TabsTrigger value="import">Import</TabsTrigger>
-            <TabsTrigger value="ldap" onClick={loadLDAP}>LDAP</TabsTrigger>
-            <TabsTrigger value="security">Security</TabsTrigger>
-            <TabsTrigger value="tenants" onClick={loadTenants}>Mandanten</TabsTrigger>
-            <TabsTrigger value="modules">Module</TabsTrigger>
+            {isSuperAdmin && <TabsTrigger value="ldap" onClick={loadLDAP}>LDAP</TabsTrigger>}
+            {isSuperAdmin && <TabsTrigger value="security">Security</TabsTrigger>}
+            {isSuperAdmin && <TabsTrigger value="tenants" onClick={loadTenants}>Mandanten</TabsTrigger>}
+            {isSuperAdmin && <TabsTrigger value="modules">Module</TabsTrigger>}
           </TabsList>
 
           {/* ── Dashboard ── */}
@@ -1166,7 +1167,8 @@ export default function AdminPage() {
                         <SelectContent>
                           <SelectItem value="user">User</SelectItem>
                           <SelectItem value="auditor">Auditor</SelectItem>
-                          <SelectItem value="admin">Admin</SelectItem>
+                          <SelectItem value="domain_admin">Domain Admin</SelectItem>
+                          {isSuperAdmin && <SelectItem value="superadmin">Superadmin</SelectItem>}
                         </SelectContent>
                       </Select>
                     </div>