Clickable product-photo thumbnail grid.

```jsx
<ProductGallery columns={4} images={[{alt:"Kugelkopf"},{alt:"E-Satz Kabel"},{alt:"Fahrzeug Heck"},{alt:"AHK Montage"}]} />
```

columns={4} for desktop row, columns={2} for the mobile 2x2 grid. Renders placeholder tiles until real photography is supplied.