fix(mui): Migrate to MUI v7 Grid component

- MUI v7 exports new Grid as default 'Grid' export (not Unstable_Grid2)
- Removed deprecated 'item' prop from Grid usage
- Changed responsive props from xs={6} sm={4} md={2} to size={{ xs: 6, sm: 4, md: 2 }}
- Resolves console warnings about deprecated Grid props
- Fixes import error that caused HTTP 500

maternal-web/app/page.tsx

@@ -1,7 +1,7 @@
 'use client';
 
 import { useState, useEffect } from 'react';
-import { Box, Typography, Button, Paper, Grid2 as Grid, CircularProgress } from '@mui/material';
+import { Box, Typography, Button, Paper, CircularProgress, Grid } from '@mui/material';
 import { AppShell } from '@/components/layouts/AppShell/AppShell';
 import { ProtectedRoute } from '@/components/common/ProtectedRoute';
 import { EmailVerificationBanner } from '@/components/common/EmailVerificationBanner';