feat: show blocked_count in broadcast admin UI

- Add blocked_count to Broadcast interface
- 4-column stats grid with blocked card (warning color)
- Show blocked count in broadcast list when > 0
- Fetch single broadcast by ID instead of listing all
- Add i18n keys for blocked/blockedShort in ru/en/zh/fa

src/locales/zh.json

@@ -969,6 +969,8 @@
       "total": "总计",
       "sent": "已发送",
       "failed": "失败",
+      "blocked": "已屏蔽",
+      "blockedShort": "屏蔽",
       "filter": "筛选",
       "message": "消息",
       "unknownAdmin": "未知",