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/pageMeta.test.ts

@@ -11,6 +11,7 @@ const EXPECTED: Record<Page, string> = {
   channels: "header.channelManager",
   playlists: "header.account.playlists",
   notifications: "inbox.navLabel",
+  files: "files.navLabel",
   downloads: "downloads.navLabel",
   stats: "header.usageStats",
   plex: "plex.navLabel",