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/api/mod.rs

@@ -18,6 +18,8 @@ pub struct SessionInfo {
     pub started_at: String,
     pub viewer_count: usize,
     pub is_streaming: bool,
+    pub is_online: bool,
+    pub is_persistent: bool,
     pub last_heartbeat: String,
     pub os_version: Option<String>,
     pub is_elevated: bool,
@@ -34,6 +36,8 @@ impl From<crate::session::Session> for SessionInfo {
             started_at: s.started_at.to_rfc3339(),
             viewer_count: s.viewer_count,
             is_streaming: s.is_streaming,
+            is_online: s.is_online,
+            is_persistent: s.is_persistent,
             last_heartbeat: s.last_heartbeat.to_rfc3339(),
             os_version: s.os_version,
             is_elevated: s.is_elevated,