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/dnscheck.go

@@ -66,12 +66,13 @@ type Domain struct {
 
 // Query describes the domain to check. ExpectedDKIM is the TXT value the panel
 // tells the operator to publish (domain.DKIMRecord.Value), so the check
-// compares DNS against the key this server actually signs with. ServerIPs comes
-// from a preceding Server check.
+// compares DNS against the key this server actually signs with. Hostname and
+// ServerIPs identify this server and come from a preceding Server check.
 type Query struct {
 	Name         string
 	Selector     string
 	ExpectedDKIM string
+	Hostname     string
 	ServerIPs    []string
 }