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/openapi/v1.json

@@ -8774,7 +8774,8 @@
           "control_state": {
             "type": "string",
             "default": "dialing"
-          }
+          },
+          "control_down_since": {}
         }
       },
       "RawChunkMetaDto": {