Trac: Prevent metanav overlapping search on Preferences page

[manhphuc]

Summary

This PR fixes a layout issue on the Trac Preferences page where the user metanav overlaps the search form.

Details

The banner layout is updated to allow wrapping, ensuring that the search form and metanav are placed on separate rows.
The fix relies on flexbox layout adjustments and avoids absolute positioning or hiding elements.

Result

• Prevents visual overlap between metanav and search
• Keeps markup unchanged
• Maintains a responsive and accessible layout
• No side effects on other Trac pages

Related

• Trac ticket: https://bbpress.trac.wordpress.org/ticket/3661

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