Support --server in catalog-next creation

[cmrigney]

What I did

Catalogs can be created using the --server flag, just like profiles.

Example usage

# Pull the Docker catalog
docker mcp catalog-next pull mcp/docker-mcp-catalog

# Create your own catalog as a subset of the Docker catalog
docker mcp catalog-next create my-namespace/dev-catalog --title "Dev Tools" --server catalog://mcp/docker-mcp-catalog/github+notion+obsidian

# Share it
docker mcp catalog-next push my-namespace/dev-catalog

Related issue

(not mandatory) A picture of a cute animal, if possible in relation to what you did

[slimslenderslacks]

lgtm