fix(01-code-splitting): convert SubscriptionGuard MUI icons to individual imports

- Converted 2 barrel imports (Lock, Upgrade) to per-file default imports

frontend/src/components/SubscriptionGuard.tsx

@@ -16,7 +16,8 @@ import {
 } from '@mui/material';
 import { useNavigate } from 'react-router-dom';
 import { useSubscriptionGuard, SubscriptionGuardOptions } from '../hooks/useSubscriptionGuard';
-import { Lock as LockIcon, Upgrade as UpgradeIcon } from '@mui/icons-material';
+import LockIcon from '@mui/icons-material/Lock';
+import UpgradeIcon from '@mui/icons-material/Upgrade';
 
 interface SubscriptionGuardProps extends SubscriptionGuardOptions {
   children: ReactNode;