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/components/NavigationProvider.dom.test.tsx

@@ -206,6 +206,18 @@ describe("NavigationProvider · Back", () => {
     expect(window.location.search).toBe("?keep=1");
   });
 
+  it("lands where a retired page's content went, and the entry names that page", () => {
+    const { result } = render();
+    act(() => result.current.setPage("plex"));
+    act(() => {
+      window.history.replaceState({ sfPage: "downloads" }, "", "/?page=downloads&keep=1");
+      window.dispatchEvent(new PopStateEvent("popstate", { state: { sfPage: "downloads" } }));
+    });
+    expect(result.current.page).toBe("files");
+    expect(window.history.state).toMatchObject({ sfPage: "files" });
+    expect(window.location.search).toBe("?page=files&keep=1");
+  });
+
   // lib/history's own listener takes a spent focus stamp off the entry it lands on; the rewrite must
   // not put it back, or the next landing there (an overlay closing) would refocus a stale item.
   it("does not put back the focus stamp the landing spent", () => {