Admin bar: Make the user avatar image circular

[fushar]

Trac ticket: https://core.trac.wordpress.org/ticket/64667

Summary

Make the user avatar in the admin bar circular as follows:

• Desktop viewport: bump the size from 18px to 20px; add border-radius: 50%
• Mobile viewport: bump the size from 26px to 28px; add border-radius: 50%

The size changes are made to make it look balanced with the rest of the icons in the admin bar.

This PR strictly updates the avatar on admin bar only. I did not update, for example, in profile.php page. I think it's better to let it display the original image without circular modification.

Desktop

Before

After

Before	After

Mobile

Before	After

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 4.7
Used for: CSS and screenshots generation.

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