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/lib/downloadQueue.dom.test.ts

@@ -0,0 +1,50 @@
+// @vitest-environment jsdom
+//
+// The queue as a tab watches it: a finish is a job that is done now and was not on the last look —
+// seen once, kept for the tray until it is closed, announced once. The first look only learns.
+import { beforeEach, describe, expect, it } from "vitest";
+import type { QueueJob } from "./api";
+import {
+  clearFinished,
+  noteQueue,
+  resetQueueWatch,
+  takeUnannounced,
+  useFinished,
+} from "./downloadQueue";
+import { renderHook } from "@testing-library/react";
+
+const job = (id: number, status: string) => ({ id, status }) as unknown as QueueJob;
+const finishedIds = () => renderHook(() => useFinished()).result.current.map((j) => j.id);
+
+beforeEach(resetQueueWatch);
+
+describe("downloadQueue · a finish", () => {
+  it("the first look is old news: what is already done is not announced", () => {
+    noteQueue([job(1, "done"), job(2, "running")]);
+    expect(takeUnannounced()).toEqual([]);
+    expect(finishedIds()).toEqual([]);
+  });
+
+  it("a job that turns done is announced once and kept, newest first", () => {
+    noteQueue([job(1, "running"), job(2, "queued")]);
+    noteQueue([job(1, "done"), job(2, "running")]);
+    noteQueue([job(1, "done"), job(2, "done")]);
+    expect(takeUnannounced().map((j) => j.id)).toEqual([1, 2]);
+    expect(takeUnannounced()).toEqual([]);
+    expect(finishedIds()).toEqual([2, 1]);
+  });
+
+  it("a job first seen done after the first look finished meanwhile (a cache hit, another device)", () => {
+    noteQueue([]);
+    noteQueue([job(5, "done")]);
+    expect(takeUnannounced().map((j) => j.id)).toEqual([5]);
+  });
+
+  it("a failure is not a finish; closing the tray forgets what finished", () => {
+    noteQueue([job(1, "running"), job(2, "running")]);
+    noteQueue([job(1, "error"), job(2, "done")]);
+    expect(finishedIds()).toEqual([2]);
+    clearFinished();
+    expect(finishedIds()).toEqual([]);
+  });
+});