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.

README.md

@@ -22,10 +22,11 @@ blocker and SponsorBlock keep working.
   tags to slice the feed by.
 - **Playlists with two-way YouTube sync** — build them locally, keep them in sync in both directions.
 - **In-app player** with resume, plus keyboard/scroll controls.
-- **Download Center** — save videos to the server with yt-dlp in a Plex-friendly layout (format
-  presets, per-user storage quota), trim / crop / split & join them in a built-in editor, play them
+- **Files** — one library for everything you keep: paste a link anywhere in the app to download it
+  with yt-dlp (format presets, per-user storage quota, the running downloads in a tray), upload your
+  own files, sort them into folders, trim / crop / split & join videos in a built-in editor, play them
   where you left off, then save to your device, share with another user, or hand out a public watch
-  link.
+  link that plays on any phone.
 - **Plex library** *(optional)* — point Siftlode at your own Plex server to browse and play its
   movies and shows alongside your feed, with watch state kept in sync.
 - **Multi-user** with per-user private state, a shared catalog, and a fair daily API-quota guard.