V1.2.0

docs: switch install/update commands to get.kretase.com

Now that get.kretase.com is live and confirmed serving the real
scripts (verified: curl https://get.kretase.com/panel returns the
actual install-panel.sh content), point the README and every script's
self-referential hints/next-step messages at it instead of the raw
GitHub URL. Also added a matching /uninstall-panel API route since
uninstall-panel.sh was already referenced but had no branded endpoint.