Use the name MPCkit

## What changed

- `public/index.html`: the app is called MPCkit in the page title, link previews, wordmark, heading, text and image descriptions.
- `README.md`, `nginx/mpckit.monoexp.com.conf`: the same in their text.

## Notes

- Lowercase stays where it's a name on disk or online: the site's address, repository names and links, and the build files (`mpckit.exe`, `mpckit.app`, `mpckit`).
- The screenshots still show "mpckit" in the window title, as the app does today.

## Testing

- Served `public/` locally and checked desktop and mobile screenshots.