A fresh panel no longer lets whoever reaches it first become admin:
registering the first account needs a one-time code the installer
prints (`serverkit setup-code` repeats it). SERVERKIT_ADMIN_EMAIL and
SERVERKIT_ADMIN_PASSWORD create the admin unattended instead.
Git apps now deploy what users expect: a repository with a compose
file deploys every service it declares, "Deploy latest" builds the
branch's newest commit, and push webhooks rebuild the containers.
Also fixes branch switching on single-branch clones, uploads of a
lone file, compose v1 port preflight, and CLI commands that ignored
their flags.
