feat(surf): print the serve URL and admin status on startup

- Add surf.Router.AdminPrefix (additive): the admin mount path and true
  when BuildRouter mounted the cabana admin, false otherwise
- serve binds with net.Listen before printing, so a busy port fails
  without a misleading URL, then prints the listen address with a
  browser URL and an admin status line: the admin URL, a warning naming
  summer admin:create when backend_users is empty or missing, or
  admin: off with the steps to add an admin controller
- The administrator count is bounded by a 2 s timeout and never fails boot
- route:list ends with the same admin status line
- surf and cabana READMEs and the console and routing docs describe the output

modules/surf/routetable.go

@@ -10,6 +10,15 @@ type RouteInfo struct {
 	Raw        bool
 }
 
+// AdminPrefix reports the cabana admin mount path set by BuildRouter. False
+// means no plugin registers an admin controller, so no admin routes exist.
+func (r *Router) AdminPrefix() (string, bool) {
+	if r == nil || !r.adminMounted {
+		return "", false
+	}
+	return r.adminPrefix, true
+}
+
 // Routes returns a defensive copy of every registered route.
 func (r *Router) Routes() []RouteInfo {
 	if r == nil {