Guest control: Cleaned methods involved in fetching/handling

- Moves guest user caching from User class to app container for
  simplicity.
- Updates test to use simpler $this->users->guest() method for
  consistency.
- Streamlined helpers to avoid function overlap for simplicity.
- Extracted user profile dropdown while doing changes.

app/App/Providers/AuthServiceProvider.php

@@ -9,6 +9,7 @@ use BookStack\Access\LdapService;
 use BookStack\Access\LoginService;
 use BookStack\Access\RegistrationService;
 use BookStack\Api\ApiTokenGuard;
+use BookStack\Users\Models\User;
 use Illuminate\Support\Facades\Auth;
 use Illuminate\Support\ServiceProvider;
 use Illuminate\Validation\Rules\Password;
@@ -65,5 +66,11 @@ class AuthServiceProvider extends ServiceProvider
         Auth::provider('external-users', function ($app, array $config) {
             return new ExternalBaseUserProvider($config['model']);
         });
+
+        // Bind and provide the default system user as a singleton to the app instance when needed.
+        // This effectively "caches" fetching the user at an app-instance level.
+        $this->app->singleton('users.default', function () {
+            return User::query()->where('system_name', '=', 'public')->first();
+        });
     }
 }