Add is_online/is_persistent for persistent agent sessions

- Sessions now track whether agent is online or offline
- Persistent agents (no support code) stay in session list when disconnected
- Dashboard shows online/offline status with color indicator
- Connect/Chat buttons disabled when agent is offline
- Agent reconnection reuses existing session

server/src/relay/mod.rs

@@ -97,7 +97,9 @@ async fn handle_agent_connection(
     }
 
     // Register the agent and get channels
-    let (session_id, frame_tx, mut input_rx) = sessions.register_agent(agent_id.clone(), agent_name.clone()).await;
+    // Persistent agents (no support code) keep their session when disconnected
+    let is_persistent = support_code.is_none();
+    let (session_id, frame_tx, mut input_rx) = sessions.register_agent(agent_id.clone(), agent_name.clone(), is_persistent).await;
 
     info!("Session created: {} (agent in idle mode)", session_id);
 
@@ -221,7 +223,8 @@ async fn handle_agent_connection(
     // Cleanup
     input_forward.abort();
     cancel_check.abort();
-    sessions_cleanup.remove_session(session_id).await;
+    // Mark agent as disconnected (persistent agents stay in list as offline)
+    sessions_cleanup.mark_agent_disconnected(session_id).await;
 
     // Mark support code as completed if one was used (unless cancelled)
     if let Some(ref code) = support_code_cleanup {