[bug/docs] [react/a2ui_explorer] Missing package.json prevents dev server from starting

Hi,
The React explorer app (renderers/react/a2ui_explorer) is missing its `package.json`. This means running `npm run dev` as mentioned in the [README.md](https://github.com/google/A2UI/blob/main/renderers/react/a2ui_explorer/README.md) will not launch the Vite dev server and nothing appears on localhost.

**Steps to reproduce:**
```
cd renderers/react/a2ui_explorer
npm install or npm run dev
```

**Expected**: Vite starts and the explorer is accessible in the browser.
**Fix**:
- Add a `package.json` file or
- Update README.md to reflect using vite as `npx vite` instead.

Can I please make the necessary change?

Navigate to the folder `renderers/react/a2ui_explorer`. Check if a `package.json` file exists. If missing, create one with the necessary scripts and dependencies to run `npm run dev` with Vite, or update the README.md to use `npx vite`. Verify the fix by running the dev server and ensuring it starts correctly.

Written by the indexing model from the issue text.