Filebrowser: Hierarchical Subdirectory Expansion

The user should able to open directories hierarchically without navigating into them but instead view them in the same explorer location, kind of like an accordion via a chevron button. This brings into question how pagination is handled in this case. One option is to add infinite scrolling and keeping the previous pages in memory. This way the user may load more and more files by scrolling down. At some point there are too many files on screen though so it'd be reasonable to make the subdirectory container scrollable.

The issue names the Filebrowser explorer and raises pagination, infinite scrolling, retained pages, and scrollable subdirectory containers, but no files or tests. Start by locating the Filebrowser entry point and existing pagination code, then clarify the interaction and pagination design before implementation. Done means hierarchical expansion works in place without navigation and the agreed large-directory behavior is covered.