chore: restructure project into backend and frontend folders

- Move all frontend code to frontend/ directory
- Add backend/ with Go project structure (cmd, internal, pkg)
- Add docker-compose.yml for orchestration

frontend/src/components/ui/index.ts

@@ -0,0 +1,5 @@
+export { Button } from './Button';
+export { Card } from './Card';
+export { Modal } from './Modal';
+export { Input } from './Input';
+export { Select } from './Select';