Expose UI workspace entrypoint

[YfengJ]

/claim #2781

Summary

• Added explicit main, types, and exports metadata for @freelanceflow/ui.
• Added a root JavaScript runtime entrypoint that exports Button and Card without relying on unresolved TypeScript source imports.
• Added root TypeScript declarations and a focused node:test import check for package-name consumers.

Validation

• npm test -w packages/ui
• node -e "import('@freelanceflow/ui').then(m=>{ console.log(Object.keys(m).sort().join(',')); console.log(typeof m.Button, typeof m.default.Card); }).catch(e=>{ console.error(e); process.exit(1); })"
• node --test apps/api/src/tests/*.test.js
• git diff --cached --check