Upgrade jvchat to React 19 (dependabot #65 needs manual migration)

[eldonm]

Dependabot #65 (bump react / react-dom / @types/react to 19) cannot auto-merge — it's a partial bump that fails at install, and a full React 19 migration is needed.

Why #65 fails

Install dies in ~10s with ERESOLVE:

While resolving: @types/react-dom@18.3.7
Found: @types/react@19.2.17
peer @types/react@"^18.0.0" from @types/react-dom@18.3.7

The PR bumps @types/react → 19 but leaves @types/react-dom at 18, whose peer requires @types/react@^18. Mismatched React type packages.

Migration scope

1. Align type packages — bump @types/react-dom → ^19 alongside @types/react@19 (the missing piece in chore(deps): bump react, react-dom and @types/react in /jvchat #65).
2. Ecosystem peers on React 18 — @assistant-ui/* (core/react/react-markdown) and the large @radix-ui/* set currently declare React 18 peers. Need to confirm React 19-compatible versions exist (or install with overrides) — otherwise ERESOLVE persists even after (1).
3. React 19 API/type changes — once install resolves, run tsc && vite build; address any React 19 breaking changes (e.g. ReactDOM.render removal already handled?, ref-as-prop, useRef arg requirement, JSX namespace changes).

Recommendation

Do this as a dedicated PR, not a dependabot auto-merge. Close/hold #65 until the ecosystem peers are React-19-ready.

Root-caused while triaging the outstanding dependabot PRs.