Loopin Chat Page Patch

Replace the matching files in the-loopin/loopin-web with the files in this archive:

• app/(public)/events/[eventId]/groups/[groupId]/chat/page.tsx
• lib/api/messages.ts
• lib/api/realtimeMessages.ts

Then run:

npm install
npm run lint
npm run build

The implementation matches the current backend contract:

• REST history: GET /v1/groups/{groupId}/messages (Spring Page response)
• SockJS endpoint: /ws
• STOMP subscribe: /topic/groups/{groupId}/messages
• STOMP publish: /app/groups/{groupId}/messages
• Payload: { "messageText": "..." }