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

frontend/src/components/clients/ClientSpeedTag.tsx

@@ -0,0 +1,26 @@
+import { Tag } from 'antd';
+
+import { SizeFormatter } from '@/utils';
+import type { ClientSpeedEntry } from '@/hooks/useClients';
+
+export type { ClientSpeedEntry };
+
+export function isActiveSpeed(speed?: ClientSpeedEntry): speed is ClientSpeedEntry {
+  return !!speed && (speed.up > 0 || speed.down > 0);
+}
+
+interface ClientSpeedTagProps {
+  speed: ClientSpeedEntry;
+}
+
+export function ClientSpeedTag({ speed }: ClientSpeedTagProps) {
+  return (
+    <Tag color="blue">
+      ↑ {SizeFormatter.speedFormat(speed.up)}
+      {' / '}
+      ↓ {SizeFormatter.speedFormat(speed.down)}
+    </Tag>
+  );
+}
+
+export default ClientSpeedTag;