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.

backend/tests/test_file_folders.py

@@ -261,3 +261,22 @@ def test_the_longest_host_has_room_for_its_folder(db, make_user):
     uid = make_user().id
     job = _job(db, uid, _asset(db, "url", f"https://{host}/v"), kind="url")
     assert _folder(db, job).source_key == f"site:{host}"
+
+
+def test_the_ready_notice_names_the_file_and_its_folder(db, make_user):
+    # The notice's "Open" goes to the file in its folder, so both ride along as data.
+    from app.downloads import pipeline
+
+    user = make_user()
+    job = _job(db, user.id, _asset(db))
+    pipeline._notify_download_done(db, user.id, "A title", job.id, kind="download")
+    db.commit()
+    note = db.scalars(select(Notification).where(Notification.user_id == user.id)).one()
+    assert note.type == "download_done"
+    assert note.data == {
+        "title": "A title",
+        "kind": "download",
+        "job_id": job.id,
+        "folder_id": job.folder_id,
+    }
+    assert job.folder_id is not None