Embed snippet ui polish fixes

What does this MR do?

Screenshots

This fixes the other UI polish issues mentioned in #28468 (closed) and does some additional css cleanup.

File icon aligmment

Gutter border problem

Non embedded snippet visual problem

problem area	zoomed in

Pixelated logo

Replace icons with updated graphics

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #28468 (closed)