Add disconnect/uninstall for persistent sessions

- Server: Add DELETE /api/sessions/:id endpoint to disconnect agents
- Server: SessionManager.disconnect_session() sends Disconnect message
- Agent: Handle ADMIN_DISCONNECT to trigger uninstall
- Agent: Add startup::uninstall() to remove from startup and schedule exe deletion
- Dashboard: Add Disconnect button in Access tab machine details
- Dashboard: Add Chat button for persistent sessions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

agent/src/main.rs

@@ -249,6 +249,19 @@ async fn run_agent(config: config::Config) -> Result<()> {
                         return Ok(());
                     }
 
+                    // Check if this is an admin disconnect (uninstall)
+                    if error_msg.contains("ADMIN_DISCONNECT") {
+                        info!("Session was disconnected by administrator - uninstalling");
+                        if let Err(e) = startup::uninstall() {
+                            warn!("Uninstall failed: {}", e);
+                        }
+                        show_message_box(
+                            "Remote Session Ended",
+                            "The remote support session has been ended by the administrator.\n\nThe agent will be removed from this computer.",
+                        );
+                        return Ok(());
+                    }
+
                     error!("Session error: {}", e);
                 }
             }