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/cabana/README.md

@@ -178,7 +178,7 @@ func (p *Plugin) AdminControllers() []pact.AdminController {
 func (p *Plugin) AdminFS() fs.FS { return adminFS }
 ```
 
-`config_list.yaml` and `config_form.yaml` reference the model files with WinterCMS paths such as `~/plugins/acme/blog/models/post/columns.yaml`. At boot, `surf.BuildRouter` calls `cabana.Activate` with the activated plugins and mounts the returned `cabana.Routes`; when no plugin registers an admin controller, `cabana.Activate` returns nil and no admin routes exist. Record and user lookups use the `*gorm.DB` that [lagoon](../lagoon/README.md) publishes on the `backpack.App`.
+`config_list.yaml` and `config_form.yaml` reference the model files with WinterCMS paths such as `~/plugins/acme/blog/models/post/columns.yaml`. At boot, `surf.BuildRouter` calls `cabana.Activate` with the activated plugins and mounts the returned `cabana.Routes`; when no plugin registers an admin controller, `cabana.Activate` returns nil and no admin routes exist. `surf.Router.AdminPrefix` reports which case applies, and the `serve` and `route:list` commands print it as an admin status line. Record and user lookups use the `*gorm.DB` that [lagoon](../lagoon/README.md) publishes on the `backpack.App`.
 
 ## API reference