feat(files): download from anywhere — a link box, a pasted link, the palette; the running downloads in a tray

- Files toolbar link box (paste button, Enter); a lone http(s) link pasted onto any page opens the one
  app-wide download dialog, which now names the link; the palette's "Download from a link…" focuses
  the box on its history entry.
- The download tray, bottom right on every page: what waits, runs, is paused or failed (reason +
  Retry, the queue's right-click / "…" menu), and what finished in this tab as "Done · Open" until
  closed; folds to a pill (per-account pref); closed, it stays shut — across a reload — until the
  work changes or something finishes. Back to top steps up over it, the page end keeps clear.
- A finish is announced once: "Downloaded: …", whether a phone plays it, Copy link + Open (toasts
  and notices may carry a second action).
- The inbox's ready notice opens the file in its folder: download_done carries job_id + folder_id.
- GET /api/downloads is the queue only (in flight, failed, finished in the last 10 minutes), with
  each row's source key.

frontend/src/components/Files.menu.dom.test.tsx

@@ -259,7 +259,7 @@ describe("Files · my file's row", () => {
     const field = await screen.findByRole("textbox", { name: "files.actions.rename" });
     fireEvent.change(field, { target: { value: "Más" } });
     fireEvent.keyDown(field, { key: "Escape" });
-    expect(screen.queryByRole("textbox")).toBeNull();
+    expect(screen.queryByRole("textbox", { name: "files.actions.rename" })).toBeNull();
     expect(update).not.toHaveBeenCalled();
     await waitFor(() => expect(rowOf("Clip 2").contains(document.activeElement)).toBe(true));
   });