fix(plex): a collection kept after a failed refresh holds its seed title

When Plex made the collection and only the read after it failed, the kept row had no members: the
title showed unticked in the collection it had just been added to, the collection empty, and the
name stayed in the field for a retry that could only be refused as taken. The seed's row now names
the collection (child_count 1), and the panel clears the field on that refusal as on success. Also
held: the panel's lines sit beside its list, and a keyboard Retry lands on the first row.

backend/tests/test_plex_collections_filter.py

@@ -360,6 +360,9 @@ def test_create_resync_failure_is_422(db, monkeypatch, override_dep):
     # where collections are added to, and the same name is not made twice.
     row = by_rating_key(db, PlexCollection, "NEW")
     assert row is not None and row.editable and row.title == "Fresh Coll"
+    # Plex made it with the seed in it: the seed's row says so, so the title shows ticked in it.
+    assert row.child_count == 1
+    assert "NEW" in (by_rating_key(db, PlexItem, "m1").collection_keys or [])
     again = client.post(
         "/api/plex/collections",
         json={"library": "1", "title": "Fresh Coll", "item_rating_key": "m1"},