[UI] Main menu buttons

[Nefariusek]

Acceptance criteria

Verify, consult and update before starting.

• Main Menu buttons should be replaced with the Button component.
• Koala appears when the mouse is over the button.
• Location should change accordingly.

Additional notes

• Use Router component

[Urszuja]

I did all, however the issue of "hide&seek" koala remains unsolved. Is it to be cleared for this issue or later?

[Urszuja]

done.