feat(files): the Files page — the library in one aligned list, its folders in a rail

Files is the one library: every done file, the ones shared with me, or one
folder's, picked in a side rail (sources, own folders, the storage meter) and
mirrored to ?folder=. One table with every kind of value in its own column;
Copy link, Share and Save in sight, every action on a right click: play,
cut, rename in the row, move to an own folder or back home, keep, details,
re-download, delete.

The Downloads page keeps only the queue until the tray replaces it. The old
/api/downloads library routes are gone: list shared, upload, the details
PATCH, delete, the file and the usage now live under /api/files, and a
listed file says which source it came from. The job's "expired" flag is gone
with the state it described.

frontend/src/lib/urlState.ts

@@ -131,6 +131,7 @@ const PAGES = [
   "audit",
   "logs",
   "notifications",
+  "files",
   "downloads",
   "plex",
 ] as const;