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.

tests/Helpers/UserRoleProvider.php

@@ -55,7 +55,7 @@ class UserRoleProvider
      */
     public function guest(): User
     {
-        return User::getDefault();
+        return User::getGuest();
     }
 
     /**