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/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;