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/routelist_command.go

@@ -11,7 +11,8 @@ import (
 )
 
 // RouteListCommand builds the router the same way ServeCommand does, then
-// renders Routes() as a table instead of serving.
+// renders Routes() as a table instead of serving, followed by the admin
+// status line.
 func RouteListCommand(app *backpack.App, plugins []party.Plugin) bonfire.Command {
 	return bonfire.Command{
 		Name:        "route:list",
@@ -33,6 +34,11 @@ func RouteListCommand(app *backpack.App, plugins []party.Plugin) bonfire.Command
 				})
 			}
 			out.Table([]string{"Method", "Pattern", "Plugin", "Middleware", "Raw"}, rows)
+			if prefix, mounted := r.AdminPrefix(); mounted {
+				out.Info("admin: mounted at " + prefix)
+			} else {
+				out.Info(adminOffText)
+			}
 			return nil
 		},
 	}