feat: 智能体与技能列表显示创建时间字段

- 新增共享 formatDateTime 工具(YYYY-MM-DD HH:mm)
- agent-card 卡片/列表视图在发布者下方显示创建时间
- skill 设置页 metaNodes 显示创建时间

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/src/core/utils/datetime.ts

@@ -25,3 +25,18 @@ export function formatTimeAgo(date: Date | string | number, locale?: Locale) {
     locale: getDateFnsLocale(effectiveLocale),
   });
 }
+
+/**
+ * Format a created/updated timestamp as `YYYY-MM-DD HH:mm`.
+ * Returns an empty string for missing or unparseable values.
+ */
+export function formatDateTime(value?: Date | string | number | null): string {
+  if (value === null || value === undefined || value === "") return "";
+  const date = value instanceof Date ? value : new Date(value);
+  if (Number.isNaN(date.getTime())) return "";
+  const pad = (n: number) => String(n).padStart(2, "0");
+  return (
+    `${date.getFullYear()}-${pad(date.getMonth() + 1)}-${pad(date.getDate())} ` +
+    `${pad(date.getHours())}:${pad(date.getMinutes())}`
+  );
+}