Add a features card for native, small and offline

## What changed

- `public/index.html`: a fifth Features card, "Native, small and offline": one small native app for Windows, macOS and Linux, built with Go; no installer, account or tracking; never connects to the internet, so samples and projects stay on the computer.
- `public/style.css`: `.card.wide` spans both columns, so the odd card sits full width under the other four instead of leaving a gap.

## Notes

- Checked against the app's code: its only HTTP is Wails' in-process asset server (pad audio from Go to the window), fonts are bundled with Fontsource, and the About repo link opens in the system browser only when clicked.
- No size in megabytes: it isn't measured yet.

## Testing

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