ProxyInstallAssistant

Stages a copy of the running universal jar for the proxy when this backend is proxied but has never been provisioned with a network key.

This restores the 2.0.0-era proxy-extension assist in universal-jar form: back then the backend bundled separate Velocity/Bungee jars and extracted them with copy instructions; 2.2.1 removed that machinery when the single universal jar made the per-platform variants obsolete, and the assisted install UX silently vanished with it. The jar being universal now makes the assist strictly simpler — there is exactly one file, and it is byte-identical to the plugin already running here, so proxy and backend can never skew versions.

The staged copy is written to plugins/ResourcePackManager/proxy-extension/ResourcePackManager.jar. Deployment onto the proxy stays a human action: cross-machine there is no credentialed channel that could place a jar remotely, and same-machine guessing at another server's directory is not safe. The handshake after deployment is fully automatic — the proxy mints the network key and pushes signed grants on player connect.