panel: show the SPF and DMARC records the domain page expects

The domain page generated and displayed the DKIM record but said only
"also configure SPF and DMARC for the domain (see the documentation)".
The concrete example existed — buried in the check's remediation text,
and only visible once the check had already failed. Show both records up
front, host and value with a Copy button, the way the DKIM record is
shown, plus the two things that actually bite: a domain may carry only
one SPF record (add the mechanism to the existing one rather than
publishing a second), and p=none is safe to publish immediately.

The SPF value names the addresses SELFPOST_HOSTNAME resolves to, taken
from the hostname check the page already runs, and falls back to an "a:"
mechanism when it does not resolve. New dnscheck.SPFExample/DMARCExample
are the single source for both the page and the checks' advice, so the
two cannot drift into recommending different records; dnscheck.Query
gains Hostname for the fallback.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/dnscheck/spf.go

@@ -47,7 +47,7 @@ func (c *Checker) checkSPF(ctx context.Context, q Query) Result {
 	case !found || len(records) == 0:
 		return Result{
 			Status: health.StatusError,
-			Detail: fmt.Sprintf("No SPF record is published for %s. Publish a TXT record such as \"v=spf1 ip4:%s -all\" — without it receivers have nothing authorising this server to send as the domain.", q.Name, ips[0]),
+			Detail: fmt.Sprintf("No SPF record is published for %s. Publish a TXT record such as %q — without it receivers have nothing authorising this server to send as the domain.", q.Name, SPFExample(q.Hostname, q.ServerIPs)),
 		}
 	case len(records) > 1:
 		return Result{