Add onboarding for first-time users

## [User story](https://www.atlassian.com/agile/project-management/user-stories)
As a **first-time visitor**, I want **to see a quick overview tutorial before logging in** so that **I can easily understand the main features and value of the app.**

## [Acceptance criteria](https://www.atlassian.com/work-management/project-management/acceptance-criteria)
- [ ] The tutorial automatically appears upon a user's first visit to the app before logging in.
- [x] The tutorial consists of 6 sequential steps/stages.
- [x] The user can freely navigate back and forth between the 6 steps.
- [x] An option to skip the entire tutorial is available at any time.
- [x] Each step displays a short, concise description accompanied by a themed cat-mascot illustration.
- [x] On mobile devices, the tutorial is displayed in full-screen mode.
- [x] On desktop devices, the tutorial is displayed as a centered modal that blocks interaction with the background content.

## Visuals (optional)
- [Figma design](https://www.figma.com/design/0kVFdH1uo5CECYEVP1jHkg/AKAI-PROJECT?node-id=0-1&p=f)

## Notes