fix(docs): make checkbox group error demo interactive

[pranavkatariain]

Summary

• Store the error-group selection in React state instead of freezing its value to an empty array.
• Clear the validation message and error styling once at least one option is selected.
• Replace the resolved error with same-height helper text so the demo layout remains stable.
• Give the centred demo group a stable responsive width so different message lengths do not move it horizontally.

Before and after

Before	After
Clicking Email does not retain the checked state.	Clicking Email displays the tick and swaps the error for helper text without moving the layout.

Verification

• pnpm --filter @cloudflare/kumo build
• pnpm exec vp lint packages/kumo-docs-astro/src/components/demos/CheckboxDemo.tsx
• pnpm --filter @cloudflare/kumo-docs-astro typecheck
• Manually verified locally that Email changes from aria-checked="false" to aria-checked="true", the error swaps to helper text, the ring returns to its default style, and the preview remains stable on both axes.

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• Reviews
  • bonk has reviewed the change
  • automated review not possible because:
• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows: clicked Email in the local error-group demo and confirmed its checked state persists while the error swaps to helper text without shifting the preview vertically or horizontally
  • Additional testing not necessary because: