web: show how long a line has been out of contact

The Backend now keeps the time since a link's control layer was last up, because retries move the control state in and out of backoff and its own timestamp keeps resetting. The overview exposes it as control_down_since and the card shows it next to the lost-contact notice.

web/display/src/pages/Overview.tsx

@@ -2,7 +2,7 @@ import { useEffect, useMemo, useRef, useState } from 'react'
 import { Link, useSearchParams } from 'react-router-dom'
 import { api } from '@shared/api/client'
 import type { AgentIpv6Dto, NodeInfoDto, OverviewDto, OverviewLineDto, Page } from '@shared/api/types'
-import { fmtAgo } from '@shared/format'
+import { fmtAgo, fmtDuration } from '@shared/format'
 import { useQuery } from '@shared/hooks'
 import { Chip, Dot, Empty, Segmented, Spinner } from '@shared/ui'
 import { fmtBytes, fmtSpeed, fmtUptime, pct, useHosts, type HostItem } from '../live'
@@ -196,7 +196,7 @@ function LineCard({ l, range, infos }: { l: OverviewLineDto; range: OverviewRang
   const connectionIssue = !l.online
     ? 'Agent 离线'
     : l.quic_health?.control_state === 'backoff'
-      ? '与 Agent 失联'
+      ? `与 Agent 失联${l.quic_health.control_down_since ? ` ${fmtDuration(Math.max(0, Date.now() - new Date(l.quic_health.control_down_since).getTime()))}` : ''}`
       : l.quic_health?.state === 'retry_wait'
         ? 'QUIC 拨号失败'
         : !l.dialer_connected

web/shared/api/schema.ts

@@ -2120,6 +2120,7 @@ export interface components {
             retry_in_ms?: null | number | string;
             /** @default dialing */
             control_state: string;
+            control_down_since?: unknown;
         };
         RawChunkMetaDto: {
             chunk_id: string;