Add accessible name to search button in narrow layout

[rezabakhshilaktasaraei]

Summary

• The search icon button shown when the filters sidebar is active had no accessible name, making it appear as an unnamed button to screen readers.
• Sets lng_dlg_filter ("Search") as the accessible name for _searchForNarrowLayout.

Test plan

• Enable filters sidebar (create a folder)
• Navigate with Tab using a screen reader — the search button should announce "Search" instead of just "button"

[john-preston]

Thanks!