Add Storybook for consent management components

[mr-tanta]

Summary

We need a Storybook setup so contributors and users can visually browse and interact with the consent management components before installing the package.

What needs to happen

• Set up Storybook 8+ in the project (npx storybook@latest init)
• Create stories for the consent module components in packages/ndpr-toolkit/src/components/consent/
• Each story should show:
  • Default state
  • With pre-filled props
  • Dark mode variant (if applicable)
  • Different consent categories (analytics, marketing, functional)
• Add a storybook script to package.json
• Update README with a link to the deployed Storybook (we can use Chromatic or Vercel for hosting later)

Acceptance criteria

• Storybook runs locally with pnpm storybook
• At least 3 stories for the consent module
• Stories render without errors
• TypeScript strict mode compatible

Helpful context

• Components live in packages/ndpr-toolkit/src/components/consent/
• The project uses TypeScript strict mode
• Styling uses Tailwind CSS + Radix UI primitives