fix(#726): handle broadcast Lagged + clear on SSE reconnect

Distinguish RecvError::Lagged from Closed in the build-log stream
loop: Lagged continues (next recv delivers the full accumulated delta
via saved cursors), Closed returns (shutdown path).

Clear pre.textContent and reset cursor lengths in es.onerror when
readyState is CONNECTING so auto-reconnect doesn't double-append the
replay from cursor=0.

hive-c0re/src/dashboard.rs

@@ -1789,7 +1789,8 @@ async fn get_build_log_stream(
                         Err(_) => return,
                     }
                 }
-                Err(_) => return, // notification channel closed (shutdown)
+                Err(tokio::sync::broadcast::error::RecvError::Lagged(_)) => continue,
+                Err(tokio::sync::broadcast::error::RecvError::Closed) => return,
             }
         }
     });