Migrate deprecated MUI Grid to the new Grid2 component (yorkie-team#512)

* Migrate deprecated MUI Grid to the new Grid2 component

This file hadn’t been updated like the others and uses the deprecated Grid API
causing warnings. So migrated it to Grid2 to match the project’s MUI v6 version.

* refactor: alias Grid2 as Grid for consistency

Unified Grid2 imports across multiple files by aliasing it as Grid,
improving seamless usage and readability.

Author: yeonthusiast

frontend/src/pages/Index.tsx

@@ -1,4 +1,4 @@
-import { Box, Container, Divider, Grid, Paper, Stack, Typography } from "@mui/material";
+import { Box, Container, Divider, Grid2 as Grid, Paper, Stack, Typography } from "@mui/material";
 import CodePairIcon from "../components/icons/CodePairIcon";
 import { GithubLoginButton } from "react-social-login-buttons";
 
@@ -30,15 +30,15 @@ function Index() {
 						</Box>
 						<Stack gap={2}>
 							<Grid container spacing={1} alignItems="center">
-								<Grid item xs>
+								<Grid size="grow">
 									<Divider sx={{ width: 1 }} />
 								</Grid>
-								<Grid item xs="auto">
+								<Grid size="auto">
 									<Typography variant="body2" color="text.secondary">
 										Login with
 									</Typography>
 								</Grid>
-								<Grid item xs>
+								<Grid size="grow">
 									<Divider sx={{ width: 1 }} />
 								</Grid>
 							</Grid>

frontend/src/pages/workspace/Index.tsx

@@ -9,7 +9,7 @@ import {
 	Box,
 	Button,
 	CircularProgress,
-	Grid2,
+	Grid2 as Grid,
 	Paper,
 	Stack,
 	Tab,
@@ -140,17 +140,17 @@ function WorkspaceIndex() {
 				}
 			>
 				<Box width={1}>
-					<Grid2
+					<Grid
 						container
 						spacing={{ xs: 2, md: 3 }}
 						columns={{ xs: 4, sm: 8, md: 12, lg: 12 }}
 					>
 						{documentList.map((document) => (
-							<Grid2 key={document.id} size={4}>
+							<Grid key={document.id} size={4}>
 								<DocumentCard document={document} />
-							</Grid2>
+							</Grid>
 						))}
-					</Grid2>
+					</Grid>
 				</Box>
 			</InfiniteScroll>
 			<CreateModal