Replace static desktop-only layout with responsive version: mobile gets a
tab bar (Recent / Upload) and a bottom-sheet drawer for file details, while
desktop retains the existing three-column panel layout. FileDetails gains
updated mobile wrapper classes (removes border/width, uses flex gap).
Creates the full Files page UI (Tasks 8 & 9): FilesNav, FileListItem,
FileList, FileDetails, FileUploadArea components, FilesPage layout, and
wires up the /files route plus sidebar/mobile-nav entries.