panel-redesign: no report mailbox for the server as a whole

Follows the design commit before it.

  - internal/dmarc: the ingest allow-list is the report address of each
    domain that is on this host and nothing else;
    DefaultHostedReportAddress and DefaultHostedSuggestion are removed.
    Since 1.7.0 the general mailbox dmarc-reports@<hostname> was open
    while no panel-wide default existed; the three-choice scheme had
    left it open always.
  - The DMARC hub's Ingest box shows the form of a hosted address,
    dmarc-reports+<domain>@<hostname>.
  - guide.md: what port 25 accepts.

Evidence: docs/assets/panel-redesign/evidence/dmarc.png re-shot (the
address form wraps after the plus sign in its column at 1440 px).

  go test -count=1 ./internal/dmarc/... ./internal/web/... ./cmd/panel: ok

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/dmarc/service.go

@@ -64,10 +64,9 @@ func (s *Service) AllowedRecipients() ([]string, error) {
 		seen[addr] = true
 		out = append(out, addr)
 	}
-	// The instance's generic mailbox is always open: it is the address the
-	// guide tells a fresh install to publish. A domain's own report address
-	// opens the mailbox it names, if that is on this host.
-	add(DefaultHostedReportAddress(s.hostname))
+	// Only what a domain has chosen is open: its report address opens the
+	// mailbox it names, if that is on this host. There is no mailbox for the
+	// server as a whole.
 	domains, err := s.store.ListDomains()
 	if err != nil {
 		return nil, err
@@ -101,11 +100,6 @@ func (s *Service) HostedSuggestion(domain string) string {
 	return HostedReportAddress(s.hostname, domain)
 }
 
-// DefaultHostedSuggestion is the settings-level hosted address.
-func (s *Service) DefaultHostedSuggestion() string {
-	return DefaultHostedReportAddress(s.hostname)
-}
-
 // ValidateHostedAddress ensures addr is on this hostname before saving.
 func (s *Service) ValidateHostedAddress(addr string) error {
 	if !IsHostedOnHostname(addr, s.hostname) {