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.yml

@@ -7,6 +7,7 @@ services:
     environment:
       - PUID=${PUID:-1000}
       - PGID=${PGID:-1000}
+      - DATA_DIR=/app/data
     volumes:
       - ./data:/app/data
     ports: