Fix secondary header menu items stacking vertically

[jjroelofs]

Linked issues

• Secondary header menu items stack vertically instead of displaying inline #793

Solution

Add flex-direction: row to #secondary-header ul.menu to override Bootstrap 5's default flex-direction: column on .navbar-nav. The fix is scoped to the secondary header only because the main navigation already uses Bootstrap's .navbar-expand-lg class which properly manages flex-direction based on viewport.

Checklist

• I have read the CONTRIBUTING.md document.
• My commit messages follow the contributing standards and style of this project.
• My code follows the coding standards and style of this project.
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Need to run update.php after code changes
• Requires a change to end-user documentation.
• Requires a change to developer documentation.
• Requires a change to QA tests.
• Requires a new QA test.
• I have updated the documentation accordingly.
• All new and existing tests passed.