fix(system): sort recent tasks by finished_at desc

「最近任务」原先按 kv 键名排序；改为按 finished_at 倒序，
缺完成时间的排末尾。前端渲染侧同步按 finished_at 排序兜底。

reaction_board/web/static/views/system.js

@@ -6,7 +6,16 @@ import {
 } from "../format.js";
 
 function taskRows(tasks) {
-  const entries = Object.entries(tasks || {});
+  const entries = Object.entries(tasks || {}).sort((a, b) => {
+    const fa = Number(a[1] && a[1].finished_at);
+    const fb = Number(b[1] && b[1].finished_at);
+    const aOk = Number.isFinite(fa);
+    const bOk = Number.isFinite(fb);
+    if (aOk && bOk) return fb - fa;
+    if (aOk) return -1;
+    if (bOk) return 1;
+    return String(a[0]).localeCompare(String(b[0]));
+  });
   if (!entries.length) return emptyState("暂无任务记录", "运行一次同步、快照或发现后会写入最近结果。");
   return `<div class="table-wrap">
     <table class="data-table">