feat: add company management and user-company assignment in admin panel

shop/app/admin/layout.tsx

@@ -1,5 +1,5 @@
 import Link from 'next/link'
-import { LayoutDashboard, Package, Settings, Users, LayoutGrid, ShoppingCart, Wrench, Database } from 'lucide-react'
+import { LayoutDashboard, Package, Settings, Users, LayoutGrid, ShoppingCart, Wrench, Database, Building2 } from 'lucide-react'
 import { redirect } from 'next/navigation'
 import { createClient } from '@/lib/supabase/server'
 import { AdminLogoutButton } from '@/components/admin/logout-menu-item'
@@ -58,6 +58,10 @@ export default async function AdminLayout({
             <Users className="w-5 h-5" />
             Benutzer
           </Link>
+          <Link href="/admin/companies" className="flex items-center gap-3 px-3 py-2 rounded-lg text-slate-400 hover:text-white hover:bg-white/5 transition-all">
+            <Building2 className="w-5 h-5" />
+            Firmen
+          </Link>
 
           <div className="pt-2 pb-1 px-3">
             <span className="text-[10px] font-semibold uppercase tracking-wider text-slate-600">Einstellungen</span>