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/keymap.ts

@@ -117,14 +117,8 @@ export const SHORTCUTS = [
     labelKey: "keys.goNotifications",
     page: "notifications",
   },
-  { id: "goFiles", scope: "page", keys: ["g l"], labelKey: "keys.goFiles", page: "files" },
-  {
-    id: "goDownloads",
-    scope: "page",
-    keys: ["g d"],
-    labelKey: "keys.goDownloads",
-    page: "downloads",
-  },
+  // "g d" is the retired Downloads page's chord: what it held is in Files now, so it goes there too.
+  { id: "goFiles", scope: "page", keys: ["g l", "g d"], labelKey: "keys.goFiles", page: "files" },
   { id: "goStats", scope: "page", keys: ["g s"], labelKey: "keys.goStats", page: "stats" },
   { id: "goSettings", scope: "page", keys: ["g ,"], labelKey: "keys.goSettings", page: "settings" },