Dense 28px action button — use for every clickable action in Atlas chrome (topbar, drawers, toolbars, card headers). Exactly one `primary` per view.

```jsx
<Button variant="primary" icon={<PlusIcon />}>New event</Button>
<Button>Matrix view</Button>
<Button variant="ghost" iconOnly icon={<MoreIcon />} />
```

Variants: `default` (white, bordered), `primary` (flush-600 fill — the single main action), `ghost` (chromeless, for low-emphasis/icon actions). Set `iconOnly` for square icon buttons. Forwards all native button props (`onClick`, `disabled`, …).