feat(files): the Downloads page goes — the tray, a Storage tab and a Downloads settings tab take its place

- The Downloads module, its rail icon and badge are gone. A bookmark, the remembered page and an old
  history entry that name it land on Files (urlState.resolvePage); `g d` goes to Files too.
- Users page: a Storage tab — the disk in numbers, every user's use against their own limit with
  their file count, every user's downloads; aligned tables, one menu per row (edit / reset quota,
  open source, copy, the owner's quota). The admin storage rows carry quota + file count, the admin
  job list its owner.
- Settings: a Downloads tab — the download formats in one list (duplicate a built-in, edit or delete
  your own) and the site sign-ins, moved from Account.
- "View" on an added download, a clip or a converting upload opens the tray on that row (unfolded,
  back if closed; a finished one opens as a file; under a player it opens without taking focus).
- The download dialog opens on the format used last.
- DataTable: `pager={false}` for a short list, `cardRow` to give a lone "…" its own card line.
- Welcome and README speak of Files; a new Files screenshot.

frontend/src/lib/storage.ts

@@ -26,6 +26,9 @@ export const LS = {
   // The queue the reader closed the tray on (job ids, a failure marked) — it stays shut until that
   // changes. Per-account mirror of the server pref.
   downloadTrayClosed: "siftlode.downloadTrayClosed",
+  // The download format used last — the download dialog opens on it. Per-account mirror of the
+  // server pref.
+  downloadProfile: "siftlode.downloadProfile",
   hints: "siftlode.hints",
   lang: "siftlode.lang",
   filters: "siftlode.filters",