fix multimodel input bars slowly being pushed offscreen (#13620)

### What problem does this PR solve?

when the conversation starts to get long on multimodel chat, the
conversation pushes the input bar offscreem

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

web/src/pages/next-chats/chat/chat-box/next-multiple-chat-box.tsx

@@ -330,7 +330,7 @@ export function MultipleChatBox({
   );
 
   return (
-    <section className="h-full flex flex-col px-5">
+    <section className="flex flex-1 min-h-0 flex-col px-5">
       <div
         className="flex gap-4 flex-1 px-5 pb-14 min-h-0"
         data-testid="chat-detail-multimodel-grid"