A2uiSurface throws on SSR: useSyncExternalStore missing getServerSnapshot

## Bug

When rendering `` in a server-rendered React app (Next.js App Router, Remix, etc.), React throws during the server pass:

```
Error: Missing getServerSnapshot, which is required for server-rendered content. Will revert to client rendering.
```

Tracked this down to `A2uiSurface` calling `useSyncExternalStore` without a `getServerSnapshot` argument. React requires this on the server to produce a stable, hydration-safe snapshot — without it the call throws on every SSR render.

## Steps to reproduce

1. Install `@a2ui/react@0.10.2` in a Next.js 14+ (App Router) project
2. Render `` anywhere that executes server-side
3. Server logs the above error on first render

## Expected vs actual

Expected: the component either renders gracefully on the server (returning a stable snapshot or `null`) or is documented as client-only with a clear pattern for opting out of SSR.

Actual: React throws during the server render pass. In strict SSR setups (e.g. `experimental.serverActions: true` or a CI render test) this surfaces as a hard error rather than a recoverable warning.

## Workaround

Skip the component server-side via a `useEffect`-guarded mount flag or a `dangerouslySetInnerHTML` fallback. Works, but means the surface is effectively non-functional until JS loads.

## Environment

- `@a2ui/react`: 0.10.2
- `react`: 19.x
- Next.js: 14 (App Router)

Find the A2uiSurface component in the codebase, likely in a file like `packages/react/src/A2uiSurface.tsx`. Look for the `useSyncExternalStore` call. Understand the React 19 SSR requirements for `getServerSnapshot`. Check if there are existing patterns for server-side rendering in the project. The fix involves providing a proper `getServerSnapshot` function or making the component client-only. Verify the fix by testing in a Next.js App Router setup.

Written by the indexing model from the issue text.