refactor(admin-lehrer): Rename to Admin Lehrer KI and remove migrated categories

Remove communication, infrastructure, and development categories from
navigation (now in Admin Core on port 3008). Rename Admin v2 to
Admin Lehrer KI in sidebar, header, and browser title.

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

admin-lehrer/components/layout/Sidebar.tsx

@@ -148,7 +148,7 @@ export function Sidebar({ onRoleChange }: SidebarProps) {
       <div className="h-16 flex items-center justify-between px-4 border-b border-slate-700">
         {!collapsed && (
           <Link href="/dashboard" className="font-bold text-lg">
-            Admin v2
+            Admin Lehrer KI
           </Link>
         )}
         <button