Redesign marketplace: plain Use button + restore library/category filters

Drop the download icon and the inline (project/global) scope label from the
plugin action button — it now just says "Use" and asks for scope afterward
via a popover (this project vs. everywhere). Restore the dashboard-style
left filter panel with a Libraries section (marketplace list, add owner/repo,
remove, refresh) and a Categories filter list.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: EfeAcar6431