chore(web): finalize scroll behavior and preview assertions

reaction_board/web/static/router.js

@@ -82,7 +82,7 @@ async function render() {
       // 导航状态更新不应阻断页面渲染
     }
   }
-  window.scrollTo({ top: 0, behavior: "instant" });
+  window.scrollTo({ top: 0, behavior: "auto" });
   routeElement.innerHTML = `<div class="page-loading"><div class="skeleton skeleton-title"></div>
     <div class="skeleton skeleton-card"></div><div class="skeleton skeleton-card"></div></div>`;
   try {

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