fix(files): a steady list header, every user on the Storage tab, the name gets the row's room

- The Files list starts at the scroller's top: the padding above it let the sticky header ride up
  12px on the first scroll, then stop.
- Users -> Storage lists every user with their quota, not only those already holding a file — a
  quota is set before anyone downloads.
- On a wide Files list only the "…" holds a place; Copy link, Share and Save come up over the date
  while the row is hovered or holds the keyboard (a card keeps them in its line). Keeps is narrower.

backend/tests/test_upload_conversion.py

@@ -593,9 +593,10 @@ def test_the_admin_storage_view_counts_a_waiting_upload(db, as_user, make_user,
     _probe(monkeypatch)
     u = make_user()
     _ingest(db, _root, u.id, size=10)
-    r = as_user(make_user(role="admin")).get("/api/admin/downloads/storage").json()
+    admin = make_user(role="admin")
+    r = as_user(admin).get("/api/admin/downloads/storage").json()
     assert (r["files_on_disk"], r["total_bytes"]) == (1, 10)
-    assert {p["user_id"]: p["footprint_bytes"] for p in r["per_user"]} == {u.id: 10}
+    assert {p["user_id"]: p["footprint_bytes"] for p in r["per_user"]} == {u.id: 10, admin.id: 0}
 
 
 def test_the_admin_storage_view_names_each_users_quota_and_files(
@@ -612,15 +613,26 @@ def test_the_admin_storage_view_names_each_users_quota_and_files(
     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)
-    ]
+    rows = admin.get("/api/admin/downloads/storage").json()["per_user"]
+    mine = [(p["files"], p["max_bytes"], p["unlimited"]) for p in rows if p["user_id"] == u.id]
+    assert mine == [(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)}
 
 
+def test_the_admin_storage_view_lists_every_user_even_one_without_a_file(db, as_user, make_user):
+    # The admin sets a quota BEFORE anyone downloads: a user holding nothing is still a row.
+    empty = make_user()
+    admin_user = make_user(role="admin")
+    admin = as_user(admin_user)
+    rows = admin.get("/api/admin/downloads/storage").json()["per_user"]
+    by_id = {p["user_id"]: p for p in rows}
+    assert {empty.id, admin_user.id} <= by_id.keys()
+    assert (by_id[empty.id]["email"], by_id[empty.id]["files"]) == (empty.email, 0)
+    assert by_id[empty.id]["footprint_bytes"] == 0
+
+
 @pytest.mark.parametrize(
     ("job_status", "asset_status", "ref_count"),
     [