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/id-ID.json

@@ -809,6 +809,7 @@
       "groupPlaceholder": "mis. customer-a",
       "comment": "Komentar",
       "traffic": "Lalu lintas",
+      "speed": "Kecepatan",
       "offline": "Offline",
       "addClient": "Tambah klien",
       "qrCode": "Kode QR",