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/addresses.go

@@ -4,7 +4,8 @@ import (
 	"strings"
 )
 
-// DefaultLocalPart is the mailbox local-part for the panel-wide hosted address.
+// DefaultLocalPart is the local part of every hosted address; the domain
+// follows it after a plus sign.
 const DefaultLocalPart = "dmarc-reports"
 
 // HostedReportAddress is the per-domain SelfPost-hosted rua= destination.
@@ -14,11 +15,6 @@ func HostedReportAddress(hostname, domain string) string {
 	return DefaultLocalPart + "+" + domain + "@" + hostname
 }
 
-// DefaultHostedReportAddress is the settings-level hosted rua= when ingest is on.
-func DefaultHostedReportAddress(hostname string) string {
-	return DefaultLocalPart + "@" + strings.ToLower(strings.TrimSpace(hostname))
-}
-
 // IsHostedOnHostname reports whether addr is delivered locally on hostname.
 func IsHostedOnHostname(addr, hostname string) bool {
 	addr = strings.ToLower(strings.TrimSpace(addr))