Refactor Button to semantic tokens and intent prop, fix error focus rings

Replace palette token references (blue-01, red-02, grey-01) with dedicated
button domain tokens. Rename `color` prop to `intent` with semantic values
(default, danger, subtle, neutral). Add loading state with spinner and
aria-busy. Fix Checkbox and Radio error states leaking teal focus ring.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/ui/Checkbox/Checkbox.tsx

@@ -64,7 +64,8 @@ export const Checkbox = forwardRef<HTMLInputElement, CheckboxProps>(
               'focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-control-focus-ring focus-visible:ring-offset-1',
               'active:scale-95',
               'disabled:pointer-events-none disabled:opacity-50',
-              hasError && 'border-control-error checked:border-control-error checked:bg-control-error',
+              hasError &&
+                'border-control-error hover:border-control-error checked:border-control-error checked:bg-control-error checked:hover:border-control-error checked:hover:bg-control-error focus-visible:ring-red-03',
             )}
             {...props}
           />