Design System: Improve icon button example accessibility (#50783)

## Problem

The Icon only button example lacks some accessibility features:
- no `aria-label` for screen readers
- no tooltip for sighted users

## Solution

Add both with comments explaining the reasons

## Review instructions

See
https://design-system-imfc534k0-supabase.vercel.app/design-system/docs/components/button#only-an-icon

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
* Clarified icon-only button guidance: use a tooltip for sighted users
and an accessible label for screen readers. When the tooltip repeats the
button’s label, prevent it from being announced twice.
* Added guidance to use a square button container and increase the tap
target by 8px. Updated the icon-button example to demonstrate a “View
logs” tooltip and accessible labeling.
* **New Features**
* Icon-only buttons now use a compact square layout with an expanded tap
target.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Danny White <3104761+dnywh@users.noreply.github.com>

packages/ui/src/components/Button/Button.tsx

@@ -119,6 +119,9 @@ const buttonVariants = cva(
       rounded: {
         true: 'rounded-full',
       },
+      iconOnly: {
+        true: 'hit-area-1 w-6.5',
+      },
     },
     // Match <Button size="tiny"> so raw buttonVariants({ variant }) keeps sizing.
     // Fixed icon shells that omit size must override padding (e.g. px-0 with h/w-[30px]).
@@ -182,7 +185,7 @@ export interface ButtonProps
   extends
     React.ButtonHTMLAttributes<HTMLButtonElement>,
     // omit 'disabled' as it is included in HTMLButtonElement
-    Omit<ButtonVariantProps, 'disabled'>,
+    Omit<ButtonVariantProps, 'disabled' | 'iconOnly'>,
     LoadingVariantProps {
   asChild?: boolean
   variant?: ButtonVariantProps['variant']
@@ -266,6 +269,7 @@ const Button = forwardRef<HTMLButtonElement, ButtonProps>(
             focusableWhenDisabled,
             block,
             rounded,
+            iconOnly: children == null || children === false,
           }),
           className
         )}