feat(admin): convert all timestamps to Beijing time (UTC+8) for display

Create utils/format.ts with toBeijing() helper. Replace 13 raw .slice().replace() date displays in admin.vue with centralized timezone-aware formatting.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

zhiyin-app/src/utils/format.ts

@@ -0,0 +1,17 @@
+/** 将 ISO 时间字符串转为北京时间（UTC+8）显示 */
+export function toBeijing(iso?: string): string {
+  if (!iso) return '--'
+  try {
+    return new Date(iso).toLocaleString('zh-CN', {
+      timeZone: 'Asia/Shanghai',
+      hour12: false,
+      year: 'numeric',
+      month: '2-digit',
+      day: '2-digit',
+      hour: '2-digit',
+      minute: '2-digit',
+    })
+  } catch {
+    return iso.slice(0, 16).replace('T', ' ')
+  }
+}