Add profanity filter in user's nickname

## Summary
Implement a basic profanity filter for the nickname update flow to prevent users from setting offensive display names. The solution should be lightweight avoiding complex external APIs or machine learning models.

## [Acceptance criteria](https://www.atlassian.com/work-management/project-management/acceptance-criteria)
- [ ] Define a simple interface (e.g., `IProfanityChecker`) to evaluate if a given string contains inappropriate words.
- [ ] Implement the interface using a local text file.
- [ ] Integrate the check into the nickname update flow. The validation must be case-insensitive and handle basic scenarios.
- [ ] If profanity is detected, the flow must stop and return a `Result.Failure` with a clear, user-friendly message.

## Notes
Don't overengineer this. We don't need now a third-party AI moderation tool at this step. A hardcoded collection of the most common offensive words in Polish and English is okay as starting point.