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.Api/Common/Mappers.cs

@@ -53,7 +53,7 @@ public static class Mappers
                 QuicState.Retrying => "retry_wait",
                 QuicState.Idle => "idle",
                 _ => "connecting",
-            }, quic.RetryAttempt, quic.RetryInMs, ControlStateName(link.Control.State));
+            }, quic.RetryAttempt, quic.RetryInMs, ControlStateName(link.Control.State), Time(snapshot.DownSinceMs));
     }
 
     private static string ControlStateName(ControlState state) => state switch

backend/src/MoShiro.Backend.Api/Contracts/PublicDtos.cs

@@ -52,7 +52,8 @@ public sealed record QuicLineHealthDto(
     string State = "connecting",
     int RetryAttempt = 0,
     long? RetryInMs = null,
-    string ControlState = "dialing");
+    string ControlState = "dialing",
+    DateTimeOffset? ControlDownSince = null);
 
 public sealed record OverviewDto(string Range, DateTimeOffset From, DateTimeOffset To, string Step, string Source, bool Degraded, IReadOnlyList<OverviewLineDto> Lines);