fix(auth): correct session check in sign-out page

Change !session to !session?.user to properly detect unauthenticated
state. useSession() returns an object, not null.

src/app/auth/signout/page.tsx

@@ -12,7 +12,7 @@ export default function SignOutPage() {
   const router = useRouter()
 
   useEffect(() => {
-    if (!session) {
+    if (!session?.user) {
       router.push("/")
     }
   }, [session, router])
@@ -22,7 +22,7 @@ export default function SignOutPage() {
     router.push("/")
   }
 
-  if (isPending || !session) {
+  if (isPending || !session?.user) {
     return null
   }