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/ClipPanel.dom.test.tsx

@@ -25,6 +25,8 @@ vi.mock("../lib/notifications", async (importOriginal) => {
   const actual = await importOriginal<typeof import("../lib/notifications")>();
   return { ...actual, notify: (n: unknown) => notify(n) };
 });
+const showInTray = vi.fn();
+vi.mock("../lib/downloadQueue", () => ({ showInTray: (id: number) => showInTray(id) }));
 const notifyActionError = vi.fn();
 vi.mock("../lib/actionErrors", async (importOriginal) => {
   const actual = await importOriginal<typeof import("../lib/actionErrors")>();
@@ -188,6 +190,11 @@ describe("ClipPanel", () => {
     expect(notify).toHaveBeenCalledWith(
       expect.objectContaining({ level: "success", message: "clip.queued" }),
     );
+    // Its View opens the tray on the new clip.
+    (
+      notify.mock.calls[notify.mock.calls.length - 1]![0] as { action: { onClick: () => void } }
+    ).action.onClick();
+    expect(showInTray).toHaveBeenCalledWith(1);
   });
 
   it("sends no audio track for a YouTube clip", async () => {