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/AppCommandPalette.dom.test.tsx

@@ -62,6 +62,8 @@ vi.mock("./savedViewsContext", () => ({
 }));
 const requestSurprise = vi.fn();
 vi.mock("../lib/homeIntent", () => ({ requestSurprise: () => requestSurprise() }));
+const requestLinkBox = vi.fn();
+vi.mock("../lib/downloadIntake", () => ({ requestLinkBox: () => requestLinkBox() }));
 
 import AppCommandPalette from "./AppCommandPalette";
 
@@ -142,6 +144,17 @@ describe("AppCommandPalette", () => {
     expect(requestSurprise).toHaveBeenCalledTimes(1);
   });
 
+  it("Download from a link takes the reader to the Files link box, asked on that entry", () => {
+    mount();
+    const item = group("actions")!.items.find((i) => i.id === "action:downloadLink")!;
+    setPage.mockImplementationOnce(() => {});
+    item.run();
+    expect(setPage.mock.lastCall?.[0]).toBe("files");
+    expect(requestLinkBox).not.toHaveBeenCalled();
+    setPage.mock.lastCall?.[1]?.();
+    expect(requestLinkBox).toHaveBeenCalledTimes(1);
+  });
+
   it("offers the debug recorder by its state", () => {
     const recorder = () => ids("actions").filter((id) => id.startsWith("action:recorder:"));
     mount();