fix: define check_domain_dns before first use and fix 2 minor issues

[Harbinder04]

Changes

1. Function ordering — check_domain_dns() was defined after it was called,
   causing check_domain_dns: command not found when LAGO_DOMAIN was
   already set in the environment. Moved the definition above its call site.
2. Missing echo — the Docker Compose install-link line was missing the
   echo command, causing a command not found error when both Docker and
   Docker Compose are missing.
3. Unquoted $pid — quoted "$pid" in spinner() to prevent globbing
   and word splitting.

Resolve: #761

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