feat: Add bottom border to top navigation

[cansuaa]

Description

Add bottom border to top navigation.

Issue: AWSUI-60155

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.42%. Comparing base (67c06fa) to head (7a3f008).
Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3192      +/-   ##
==========================================
+ Coverage   96.40%   96.42%   +0.01%     
==========================================
  Files         784      787       +3     
  Lines       22144    22195      +51     
  Branches     7191     7615     +424     
==========================================
+ Hits        21349    21401      +52     
+ Misses        788      742      -46     
- Partials        7       52      +45     

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