Add server creation UI + user-server access assignment (step 3)

New admin-only page at /servers-settings (templates/servers.html) lists
servers and creates new ones via POST /servers (ID/name/interface/
addresses/port, key pair generated server-side). Nav gets a "Servers"
link.

templates/users_settings.html gains a multi-select "Server Access"
field wired to the server_ids support added to create-user/update-user
in the previous commit, so admins can now actually assign non-admin
users to specific servers through the UI.

router/router.go

@@ -111,6 +111,11 @@ func New(tmplDir fs.FS, extraData map[string]interface{}, secret [64]byte) *echo
 		log.Fatal(err)
 	}
 
+	tmplServersString, err := util.StringFromEmbedFile(tmplDir, "servers.html")
+	if err != nil {
+		log.Fatal(err)
+	}
+
 	// create template list
 	funcs := template.FuncMap{
 		"StringsJoin": strings.Join,
@@ -125,6 +130,7 @@ func New(tmplDir fs.FS, extraData map[string]interface{}, secret [64]byte) *echo
 	templates["status.html"] = template.Must(template.New("status").Funcs(funcs).Parse(tmplBaseString + tmplStatusString))
 	templates["wake_on_lan_hosts.html"] = template.Must(template.New("wake_on_lan_hosts").Funcs(funcs).Parse(tmplBaseString + tmplWakeOnLanHostsString))
 	templates["about.html"] = template.Must(template.New("about").Funcs(funcs).Parse(tmplBaseString + aboutPageString))
+	templates["servers.html"] = template.Must(template.New("servers").Funcs(funcs).Parse(tmplBaseString + tmplServersString))
 
 	lvl, err := util.ParseLogLevel(util.LookupEnvOrString(util.LogLevel, "INFO"))
 	if err != nil {