feat(ui): client-realtime-speed (#5687)

* refactor(inbounds): extract TRAFFIC_POLL_INTERVAL_S to shared util

* feat(clients): derive per-client live speed from traffic WebSocket deltas

* feat(clients): render speed column and mobile card line

* i18n(clients): add pages.clients.speed key to all 13 locales

internal/web/translation/zh-CN.json

@@ -809,6 +809,7 @@
       "groupPlaceholder": "如 customer-a",
       "comment": "备注",
       "traffic": "流量",
+      "speed": "速度",
       "offline": "离线",
       "addClient": "添加客户端",
       "qrCode": "二维码",