Extract duplicated file tree types, helpers (buildFileTree, countFiles,
collectAllPaths, parseFrontmatter), and visual tree component into a
shared PackageFileTree component. Both import and export pages now use
the same underlying tree with consistent alignment and styling.
Import-specific behavior (action badges, unchecked opacity) is handled
via renderFileExtra and fileRowClassName props. Also removes the file
count subtitle from the import sidebar to match the export page.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
