Add filter bar focus shortcut

What does this MR do and why?

This is part of a bigger effort to implement a file tree browser (FTB) in the repository.

As a result of this MR we should: Enable Keyboard shortcut f to focus on the filter bar (!192203 (merged))

References

Screen recordings

Screen_Recording_2025-07-30_at_11.39.42_AM

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In rails console enable the feature flag

   Feature.enable(:repository_file_tree_browser)

or navigate to http://gdk.test:3443/rails/features and enable the repository_file_tree_browser feature flag

1. Visit a project such as gitlab-test
2. Navigate to the side menu Code -> Repository ex: https://gdk.test:3443/gitlab-org/gitlab-test/-/tree/master

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #545036 (closed)