Remove unused hsluv and react-intersection-observer dependencies #8723

Part of #8676 (P2).

Problem

packages/react/package.json lists two dependencies that production code doesn't use:

• hsluv: nothing in the repo imports it.
• react-intersection-observer: only PageLayout/PageLayout.test.tsx and SplitPageLayout/SplitPageLayout.test.tsx import it.

Both add to consumers' install size. They don't affect bundle size.

history looks removable because only type imports use it. Keep it: its types appear in public props, such as to?: To in Breadcrumbs.tsx:363.

Proposed fix

• Remove hsluv from dependencies.
• Move react-intersection-observer to devDependencies.
• Update package-lock.json.

Acceptance criteria

• npm run build, npm test and npm run type-check pass.
• A changeset explains the dependency change.