Add Docker Compose project scaffold

Sets up the full monorepo structure with:
- Multi-stage Dockerfile (client-build + production stages)
- docker-compose.yml for production, docker-compose.dev.yml overlay for development
- Express server (port 3000) with pg pool, health route, and SPA static file serving
- React 18 + Vite client with react-router-dom v6, nav bar, and placeholder page components
- .env.example with PostgreSQL and app config
- Empty db/migrations/ directory for future migrations
- CLAUDE.md updated with development workflow commands

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/pages/PaycheckView.jsx

@@ -0,0 +1,5 @@
+function PaycheckView() {
+  return <div><h1>Paycheck View</h1><p>Placeholder — coming soon.</p></div>;
+}
+
+export default PaycheckView;