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.

backend/src/MoShiro.Backend.Application/Agents/LinkRuntime.cs

@@ -3,7 +3,8 @@ using MoShiro.Protocol;
 
 namespace MoShiro.Backend.Application;
 
-public sealed record ReportedLink(LinkHealth Health, long AtMs);
+/// <summary><paramref name="DownSinceMs"/>：控制层从什么时候起一直不是在线（Unix 毫秒）；在线时为空。重试会让控制层状态反复进出退避，所以由 Backend 连着记，不用状态自己的时间。</summary>
+public sealed record ReportedLink(LinkHealth Health, long AtMs, long? DownSinceMs = null);
 
 /// <summary>Latest per-line control and task health reported by dialing nodes. Volatile runtime state, not historical metrics.</summary>
 public sealed class LinkRuntime
@@ -20,7 +21,8 @@ public sealed class LinkRuntime
         {
             var key = (dialerId, link.ListenerId, link.IpVersion);
             current.Add(key);
-            _links[key] = new ReportedLink(link, atMs);
+            long? downSince = link.Control.State == ControlState.Up ? null : _links.TryGetValue(key, out ReportedLink? previous) && previous.DownSinceMs is { } since ? since : atMs;
+            _links[key] = new ReportedLink(link, atMs, downSince);
         }
 
         foreach ((string Dialer, string Listener, string IpVersion) key in _links.Keys.Where(k => k.Dialer == dialerId && !current.Contains(k)))
@@ -41,7 +43,7 @@ public sealed class LinkRuntime
             old.Control.RetryInMs = null;
             old.Task.Quic.State = QuicState.Idle;
             old.Task.Quic.RetryInMs = null;
-            _links[pair.Key] = new ReportedLink(old, atMs);
+            _links[pair.Key] = new ReportedLink(old, atMs, pair.Value.DownSinceMs ?? atMs);
         }
     }
 }