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.

backend/tests/test_upload_conversion.py

@@ -598,6 +598,29 @@ def test_the_admin_storage_view_counts_a_waiting_upload(db, as_user, make_user,
     assert {p["user_id"]: p["footprint_bytes"] for p in r["per_user"]} == {u.id: 10}
 
 
+def test_the_admin_storage_view_names_each_users_quota_and_files(
+    db, as_user, make_user, _root, monkeypatch
+):
+    # A row says how full the user is against THEIR limit (a custom one, here), and how many files
+    # their Files lists — the finished ones, not one still waiting for its conversion.
+    _probe(monkeypatch)
+    u = make_user()
+    done, _ = _ingest(db, _root, u.id, size=10)
+    waiting, _ = _ingest(db, _root, u.id, name="b.mov", size=5)
+    done.status, waiting.status = "done", "queued"
+    db.commit()
+    admin = as_user(make_user(role="admin"))
+    put = admin.put(f"/api/admin/downloads/quota/{u.id}", json={"max_bytes": 1000})
+    assert put.status_code == 200
+    row = admin.get("/api/admin/downloads/storage").json()["per_user"]
+    assert [(p["user_id"], p["files"], p["max_bytes"], p["unlimited"]) for p in row] == [
+        (u.id, 1, 1000, False)
+    ]
+    # Every download names its owner, so its row can open THEIR quota.
+    jobs = admin.get("/api/admin/downloads").json()
+    assert {(j["id"], j["user_id"]) for j in jobs} == {(done.id, u.id), (waiting.id, u.id)}
+
+
 @pytest.mark.parametrize(
     ("job_status", "asset_status", "ref_count"),
     [