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/components/ui/pressableSelectable.contract.test.ts

@@ -17,6 +17,7 @@ const files = readSources(__dirname, [
 
 /** file → [how many plain Pressables, why they are not titles]. */
 const PLAIN: Record<string, [number, string]> = {
+  "FilesRail.tsx": [1, "the load-error retry"],
   "Home.tsx": [
     2,
     "the hero's picture and a shelf tile's picture (each named by aria-label; its title sits outside)",