[Overlay] model icons and buttons in chat header should be 18 px

[VolhaBazhkova]

EPAM AI DIAL chat version

0.15.0

What happened?

https://www.figma.com/design/lwL5jneveZczAj5RLcLFH2/DIAL-Chat?node-id=5613-9376&t=WWeAaI5gnwoUXcB4-4

Now it's 20 px -> bug Expected results: 18 px

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