Square icon-only button for nav toggles, close actions and inline icon actions. Always pass `label` for accessibility.

```jsx
<IconButton label="Menu"><MenuIcon /></IconButton>
<IconButton label="Approve" variant="solid"><CheckIcon /></IconButton>
```

Variants: `ghost` (bordered, default), `solid` (navy), `soft` (sunken tint). Sizes `sm`/`md`/`lg`. Keep glyphs at ~50–60% of the button dimension.