chore(web): finalize scroll behavior and preview assertions

tests/test_preview_seed.py

@@ -165,6 +165,7 @@ def test_seed_preview_end_to_end():
         )
         assert summary["ok"] is True
         assert summary["snapshots_failed"] == 3
+        assert summary["snapshots_missing"] > 0
         assert summary["date_from"] == START
         assert summary["date_to"] == LATEST
         assert summary["videos"] == 40