Node.js 25 support and smarter startup

Added Node.js 25 support to the egg and updated supported versions.
Added package manager auto-detection (npm, pnpm, yarn) with manual override via PACKAGE_MANAGER.
Improved startup and install flow for both dev and production modes.
Updated documentation with setup guidance and troubleshooting.

Full Changelog: https://github.com/joopdev/nextjs-egg-pterodactyl/commits/v1.1.0