fix: expose @freelanceflow/ui runtime entrypoints

[Daisuke134]

Summary

• replace the TypeScript-only package entrypoint with real ESM JavaScript entrypoints
• add explicit exports for the root package plus Button/Card subpaths
• preserve consumer typings with declaration files and record the peer React dependency

Verification

• npm install
• node --input-type=module -e "const mod = await import('@freelanceflow/ui'); console.log(Object.keys(mod).sort().join(','))"
• node --input-type=module -e "const { Button } = await import('@freelanceflow/ui/Button'); const { Card } = await import('@freelanceflow/ui/Card'); console.log(typeof Button, typeof Card)"
• npx tsc --noEmit packages/ui/index.d.ts packages/ui/Button.d.ts packages/ui/Card.d.ts

Closes #2781