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/api/downloads.ts

@@ -112,6 +112,8 @@ export interface DownloadJob {
   expires_at: string | null;
   asset: DownloadAsset | null;
   shared?: boolean;
+  // Whose it is — only the admin Storage tab's listing of every user's downloads carries them.
+  user_id?: number;
   user_email?: string;
   // Where the viewer stopped in it (whole seconds; 0 = from the start). Only the two library listings
   // (`downloads`, `downloadsShared`) carry it — a job a mutation returns does not.
@@ -147,12 +149,23 @@ export interface ShareLink {
 // languages is exactly the drift that CLAUDE.md's comment-vs-code rule is about.
 export const DISPLAY_NAME_MAX = 255;
 
+/** One user's row on the admin Storage tab: their footprint against their own limit, and how many
+ *  files their Files lists. */
+export interface AdminStorageUser {
+  user_id: number;
+  email: string | null;
+  footprint_bytes: number;
+  files: number;
+  max_bytes: number;
+  unlimited: boolean;
+}
+
 export interface AdminStorage {
   files_on_disk: number;
   total_bytes: number;
   total_assets: number;
   total_cap_bytes: number;
-  per_user: { user_id: number; email: string | null; footprint_bytes: number }[];
+  per_user: AdminStorageUser[];
 }
 
 export interface AdminDownloadQuota {