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/fa-IR.json

@@ -809,6 +809,7 @@
       "groupPlaceholder": "مثلاً customer-a",
       "comment": "توضیحات",
       "traffic": "ترافیک",
+      "speed": "سرعت",
       "offline": "آفلاین",
       "addClient": "افزودن کلاینت",
       "qrCode": "کد QR",