fix(registry): scale the provider mark's radius with its size (#116)

The registry's `provider-mark` clipped its artwork to a fixed 6px
corner. That is 37.5% of the 16px
mark the connect dialog and the domain field draw, so a host's squircle
tile came out a circle at
that size and kept its intended shape only at 32px. The radius is a
share of the mark now.

```diff
-  "inline-flex size-8 shrink-0 items-center justify-center overflow-hidden rounded-md ..."
+  "inline-flex size-8 shrink-0 items-center justify-center overflow-hidden rounded-[25%] ..."
```

The docs page now says how to ask for a compact mark, which is the size
that showed the defect:

```tsx
<ProviderMark className="size-4" label="Cloudflare">
  <img alt="" src="https://integrations.sh/logo/cloudflare.com?sz=64" />
</ProviderMark>
```

Samva already carries this shape in its installed copy (samva#1311); the
next registry sync would
have reverted it.

## Screenshots

![provider-mark at 16px and 32px, magnified: a fixed 6px radius rounds
the small mark into a circle, a 25% radius keeps one
shape](https://github.com/user-attachments/assets/8b0c4b43-9bab-4a4e-86d4-6691174a9ed5)

Both columns render the item through a real shadcn `cn` (clsx +
tailwind-merge) with a Tailwind
build, at 16px and 32px, then magnified to the same on-screen size so
the corners can be compared.
The docs preview kit cannot stand in for this: its `cn` joins classes
without merging them, so a
`size-4` override loses to the item's own `size-8` and every mark there
renders at 32px.

## Validation

- `bun run lint`, `bun run format:check`, `bun run typecheck`
- `apps/docs`: `bun run typecheck`, `bun run test` (6 browser specs),
`bun run build`,
`blume validate --strict`, `bun run audit --strict`, `bun run
registry:check`
- `bun run release:check`; no package changed, so no Tegami entry

https://claude.ai/code/session_017P54ng7iisz1dsk18u1w1x