fix(i18n): Add language selector to platform UI

Restore the LanguageSelector component to the platform layout header,
allowing platform users to switch languages.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/src/layouts/PlatformLayout.tsx

@@ -5,6 +5,7 @@ import { User } from '../types';
 import PlatformSidebar from '../components/PlatformSidebar';
 import UserProfileDropdown from '../components/UserProfileDropdown';
 import NotificationDropdown from '../components/NotificationDropdown';
+import LanguageSelector from '../components/LanguageSelector';
 import TicketModal from '../components/TicketModal';
 import FloatingHelpButton from '../components/FloatingHelpButton';
 import { useTicket } from '../hooks/useTickets';
@@ -77,6 +78,7 @@ const PlatformLayout: React.FC<PlatformLayoutProps> = ({ user, darkMode, toggleT
           </div>
 
           <div className="flex items-center gap-4">
+            <LanguageSelector />
             <button
               onClick={toggleTheme}
               className="p-2 text-gray-400 hover:text-gray-600 dark:hover:text-gray-200 transition-colors"