fix: make npm available in php runtime and align entrypoint shell

[vitormattos]

Summary

This PR fixes the issue where npm was not available during container bootstrap in the PHP service.

Changes

• Replace NVM-based Node setup in PHP image with Node/NPM from official node:20-bookworm-slim image
• Create explicit symlinks for npm and npx
• Run entrypoint with bash instead of sh
• Make environment checks in entrypoint shell-safe and use exec php-fpm

Validation

• docker compose build akaunting.php
• docker compose exec -u www-data akaunting.php node --version

Closes #8