The runtime warning (warnSNIMismatch) and the diagnostic command
(doctor checkSecretHost) previously implemented the same SNI-DNS
check with different logic: the runtime path was tightened in #461
to require every detected IP family to match, but the doctor still
accepted any single match. The two now agree.

Changes:

- Extract the shared check into internal/cli/sni_check.go, returning
  the resolved addresses and a per-family match status.
- Rewrite warnSNIMismatch and checkSecretHost on top of the helper.
- Doctor output now reports the mismatched IP family (IPv4, IPv6, or
  both) and lists the server's public IP alongside the DNS result.
- getIP falls back through a short list of public-IP endpoints
  (ifconfig.co, icanhazip.com, ifconfig.me) instead of relying on
  a single third-party service.

On some servers ifconfig.co is unreachable (e.g. Hetzner, AdGuard DNS
blocklists), causing 'mtg doctor' SNI-DNS check and 'mtg access' link
generation to fail. New config options allow specifying public IPs
manually, with automatic detection as fallback.

Fixes #405