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>

server/package.json

@@ -0,0 +1,18 @@
+{
+  "name": "budget-server",
+  "version": "1.0.0",
+  "main": "src/index.js",
+  "scripts": {
+    "start": "node src/index.js",
+    "dev": "nodemon src/index.js"
+  },
+  "dependencies": {
+    "cors": "^2.8.5",
+    "dotenv": "^16.4.5",
+    "express": "^4.19.2",
+    "pg": "^8.11.5"
+  },
+  "devDependencies": {
+    "nodemon": "^3.1.0"
+  }
+}