fix: auto-detect data directory in monorepo without DATA_DIR env var (#117)

- getDataDir() now detects monorepo by checking for ../docker-compose.yml
- DATA_DIR env var removed from .env and .env.example (no longer needed for local dev)
- Docker compose files explicitly set DATA_DIR=/app/data for containers
- Updated tests for monorepo detection logic

docker-compose.dev.yml

@@ -9,6 +9,8 @@ services:
       - ./data:/app/data
     env_file:
       - .env
+    environment:
+      - DATA_DIR=/app/data
     ports:
       - "3000:3000"
     security_opt: