feat(compose): add import from base64 in create service dropdown

Adds an "Import" option to the Create Service dropdown that lets users
paste a base64-encoded compose export, preview the template (compose YAML,
domains, envs, mounts) before confirming, and create the service only on
confirm. Adds a `previewTemplate` tRPC procedure that processes the base64
without touching the DB, with server access validation via session.