refactor: move integrations overview from main nav into admin area

Remove the top-level "Integrationen" sidebar entry and add an "Uebersicht"
tab to the admin integrations page with summary stats and integration cards.

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

apps/web/src/components/layout/sidebar.tsx

@@ -25,7 +25,6 @@ import {
   Network,
   Building2,
   UserPlus,
-  MessageSquare,
   type LucideIcon,
 } from 'lucide-react';
 
@@ -106,30 +105,6 @@ const mainNavItems: NavItem[] = [
       },
     ],
   },
-  {
-    key: 'integrations',
-    href: '/integrations',
-    icon: Plug,
-    requiredRoles: ['manager', 'admin'],
-    children: [
-      {
-        key: 'overview',
-        href: '/integrations',
-        icon: Plug,
-        exactMatch: true,
-      },
-      {
-        key: 'plentyOne',
-        href: '/integrations/plentyone',
-        icon: Building2,
-      },
-      {
-        key: 'zulip',
-        href: '/integrations/zulip',
-        icon: MessageSquare,
-      },
-    ],
-  },
 ];
 
 const bottomNavItems: NavItem[] = [