Add optional inbound relay (backup-MX) behind INBOUND_RELAY_ENABLE.

Port 25 accepts only configured domains and listed recipients, then forwards to an upstream; the outbound path is unchanged when the flag is off.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/web/view/templates/layout.html

@@ -70,6 +70,9 @@
     {{if eq .Active "status"}}<span aria-current="page">{{template "icon-status"}}Status</span>{{else}}<a href="/status">{{template "icon-status"}}Status</a>{{end}}
     {{end}}
     {{if eq .Active "domains"}}<span aria-current="page">{{template "icon-domains"}}Domains</span>{{else}}<a href="/domains">{{template "icon-domains"}}Domains</a>{{end}}
+    {{if and .IsGlobal .InboundEnabled}}
+    {{if eq .Active "inbound"}}<span aria-current="page">{{template "icon-inbound"}}Inbound</span>{{else}}<a href="/inbound">{{template "icon-inbound"}}Inbound</a>{{end}}
+    {{end}}
     {{if eq .Active "deliveries"}}<span aria-current="page">{{template "icon-deliveries"}}Deliveries</span>{{else}}<a href="/deliveries">{{template "icon-deliveries"}}Deliveries</a>{{end}}
     {{if .IsGlobal}}
     {{if eq .Active "mail_queue"}}<span aria-current="page">{{template "icon-mail-queue"}}Mail queue</span>{{else}}<a href="/mail-queue">{{template "icon-mail-queue"}}Mail queue</a>{{end}}
@@ -112,6 +115,7 @@
      templates so the nav above stays one readable line per page. */}}
 {{define "icon-status"}}<svg class="icon" viewBox="0 0 16 16" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M1.25 8.5h2.9L6.2 3.4l3.1 9.4 1.9-4.3h3.55"/></svg>{{end}}
 {{define "icon-domains"}}<svg class="icon" viewBox="0 0 16 16" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><circle cx="8" cy="8" r="6.25"/><path d="M1.9 8h12.2"/><path d="M8 1.75c1.85 1.8 2.8 4 2.8 6.25S9.85 12.45 8 14.25C6.15 12.45 5.2 10.25 5.2 8S6.15 3.55 8 1.75Z"/></svg>{{end}}
+{{define "icon-inbound"}}<svg class="icon" viewBox="0 0 16 16" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M2.5 9.5h11"/><path d="M8 2.75v6.2"/><path d="M5.4 6.4 8 9.05 10.6 6.4"/><path d="M3.2 12.6h9.6"/></svg>{{end}}
 {{define "icon-deliveries"}}<svg class="icon" viewBox="0 0 16 16" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M14.25 1.75 1.6 6.6l5 2.05 2.05 5z"/><path d="M14.25 1.75 6.6 8.65"/></svg>{{end}}
 {{define "icon-mail-queue"}}<svg class="icon" viewBox="0 0 16 16" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M1.75 9.5h3.3l1 1.75h3.9l1-1.75h3.3v3.05a1.2 1.2 0 0 1-1.2 1.2H2.95a1.2 1.2 0 0 1-1.2-1.2z"/><path d="M1.75 9.5 3.4 3.2a1.25 1.25 0 0 1 1.2-.95h6.8a1.25 1.25 0 0 1 1.2.95l1.65 6.3"/></svg>{{end}}
 {{define "icon-system-log"}}<svg class="icon" viewBox="0 0 16 16" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M3.75 1.75h5.1l3.4 3.4v8.05a1.05 1.05 0 0 1-1.05 1.05H3.75a1.05 1.05 0 0 1-1.05-1.05V2.8a1.05 1.05 0 0 1 1.05-1.05Z"/><path d="M8.85 1.75v3.4h3.4"/><path d="M5.35 8.6h5.3M5.35 11.1h3.5"/></svg>{{end}}