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/dashboard.go

@@ -45,11 +45,14 @@ func newDashboardTemplate() *template.Template {
 
 // dashboardData is the data passed to the dashboard template. Hostnames
 // and DomainRecords split the records in Snapshot.Hostnames, which also
-// holds the apex domains' own (see splitHostnames).
+// holds the apex domains' own (see splitHostnames). Ports holds
+// Snapshot.Ports with each port's names split into domains and
+// hostnames, as /api/v1/status gives them (see buildPorts).
 type dashboardData struct {
 	Snapshot      state.Snapshot
 	Hostnames     map[string]*state.HostnameState
 	DomainRecords map[string]*state.HostnameState
+	Ports         map[string]*statusPortInfo
 	Alerts        []notify.AlertEntry
 	StateAge      string
 	GeneratedAt   string
@@ -71,6 +74,7 @@ func (h *Handlers) HandleDashboard() http.HandlerFunc {
 			Snapshot:      snap,
 			Hostnames:     hostnames,
 			DomainRecords: domainRecords,
+			Ports:         buildPorts(snap),
 			Alerts:        alerts,
 			StateAge:      relTime(snap.LastUpdated),
 			GeneratedAt:   time.Now().UTC().Format("2006-01-02 15:04:05"),