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/web/view/dmarc.go

@@ -112,7 +112,7 @@ func NewDMARCHub(m Meta, in IngestInput) *DMARCHub {
 		IngestFacts: []Fact{
 			last,
 			{Label: "This week", Value: Plain(FormatCount(int64(in.KeptThisWeek)) + " kept · " + plural(int64(in.ParseFailures), "parse failure", "parse failures"))},
-			{Label: "Hosted address", Value: Plain(in.Hosted), Mono: true},
+			{Label: "Hosted addresses", Value: Plain(in.Hosted), Mono: true},
 			{Label: "Retention", Value: Plain(FormatCount(int64(in.RetentionMax)) + " reports or " + plural(int64(in.RetentionDays), "day", "days"))},
 		},
 		Recent: Box{No: "02", Title: "Recent reports"},