Fix #1710: clarify DKIM hostname entries

[Keegil]

Enhancement — Improves DKIM DNS setup instructions to reduce user error when entering records in domain provider interfaces.

Problem

The DKIM hostname table displayed full qualified names including the domain suffix (e.g. selector._domainkey.domainyouown.com). Many DNS provider interfaces expect only the subdomain portion without the root domain, so users were inadvertently duplicating the domain when copy-pasting the entry. Issue #1710.

Fix

• Strip the domain suffix from DKIM hostname values shown in the provider-facing table so users see only the subdomain portion to paste
• Retain the full qualified name in CNAME target values and zone-file examples, where the full form is correct
• Add a helper note that the exact format required varies by provider, with a pointer to Runbox or DNS provider support

Testing

• Unit test added for the hostname-formatting helper function
• npx tsc -p src/tsconfig.spec.json --noEmit passes

Closes #1710