dashboard and status API list a port's domains apart from its hostnames (closes #245)

A port entry in the state saves the apex domains that resolve to its
address with its hostnames. The dashboard's Ports table now has a
Domains column next to Hostnames, and a port entry in /api/v1/status
has a `domains` list, with `hostnames` no longer holding a domain. A
name is taken as a domain when it has a domain entry, as the dashboard
and API already tell a domain's own records from a hostname's. Both
read the split from one function, buildPorts. The state file is
unchanged; README says its port `hostnames` include domains.

Model: opus-5-5

internal/handlers/templates/dashboard.html

@@ -157,19 +157,20 @@
         >
           Ports
         </h2>
-        {{ if .Snapshot.Ports }}
+        {{ if .Ports }}
         <div class="overflow-x-auto">
           <table class="w-full text-left text-xs">
             <thead>
               <tr class="text-slate-500 uppercase tracking-wider">
                 <th class="py-2 px-3">Address</th>
                 <th class="py-2 px-3">State</th>
+                <th class="py-2 px-3">Domains</th>
                 <th class="py-2 px-3">Hostnames</th>
                 <th class="py-2 px-3">Checked</th>
               </tr>
             </thead>
             <tbody class="divide-y divide-slate-800">
-              {{ range $key, $ps := .Snapshot.Ports }}
+              {{ range $key, $ps := .Ports }}
               <tr class="hover:bg-surface-800/50">
                 <td class="py-2 px-3 text-slate-200 font-medium">
                   {{ $key }}
@@ -187,6 +188,9 @@
                   >
                   {{ end }}
                 </td>
+                <td class="py-2 px-3 text-slate-400 break-all">
+                  {{ joinStrings $ps.Domains ", " }}
+                </td>
                 <td class="py-2 px-3 text-slate-400 break-all">
                   {{ joinStrings $ps.Hostnames ", " }}
                 </td>