Add Next.js demo app

[sjoerdbeentjes]

Adds a minimal Next.js (App Router) demo app under apps/ that consumes @surfnet/react as a real workspace dependency, smoke-testing that the package imports and renders outside Storybook.

Changes

• New @surfnet/react-app at apps/react-app/: renders the Button and imports the package's compiled @surfnet/react/styles.css.
• apps/* added to the pnpm workspace; Turbo wires @surfnet/react-app#build after @surfnet/react#build via ^build.
• transpilePackages: ['@surfnet/react'] plus outputFileTracingRoot pinned to silence the parent-lockfile warning.
• sharp: true added to the workspace allowBuilds policy (pulled in by Next).
• README and AGENTS.md document the demo app.

Verification

• pnpm --filter @surfnet/react-app build passes (static prerender).
• Prod server renders the button with its full @surfnet/react variant classes.

Note: requires Node 22 (nvm use); Node 24 breaks corepack/pnpm in this repo.