⬆️(frontend) bump AI SDK to new version

[qbey]

Vercel's AI SDK v5 will have some breaking changes which we need to follow:

• bump to "@ai-sdk/react": "2.0.4" (also requires "zod": "3.25.76")
• update backend to switch to SSE : https://ai-sdk.dev/docs/ai-sdk-ui/stream-protocol
• update useChat use, message part use, ...

Check on https://github.com/vercel/ai/commits/main/examples/next-fastapi to see some migration actions and https://github.com/vercel/ai/blob/main/content/docs/08-migration-guides/26-migration-guide-5-0.mdx for the whole guide.

[qbey]

Preparation done on #12