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/storage.ts

@@ -10,6 +10,12 @@ export const LS = {
   plexLayout: "siftlode.plexLayout",
   playlistsLayout: "siftlode.playlistsLayout",
   playlistsCollapsed: "siftlode.playlistsCollapsed",
+  filesLayout: "siftlode.filesLayout",
+  filesCollapsed: "siftlode.filesCollapsed",
+  // The Files view last open (every file, shared with me, or a folder id), per-account.
+  filesView: "siftlode.filesView",
+  // The Files table's sort and rows-per-page (DataTable's persist blob).
+  filesTable: "siftlode.filesTable",
   hints: "siftlode.hints",
   lang: "siftlode.lang",
   filters: "siftlode.filters",