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/session/mod.rs

@@ -340,10 +340,14 @@ impl SessionManager {
 
             Some(message::Payload::Disconnect(disc)) => {
                 tracing::info!("Disconnect requested: {}", disc.reason);
-                // Check if this is a cancellation
+                // Check if this is a cancellation (support session)
                 if disc.reason.contains("cancelled") {
                     return Err(anyhow::anyhow!("SESSION_CANCELLED: {}", disc.reason));
                 }
+                // Check if this is an admin disconnect (persistent session)
+                if disc.reason.contains("administrator") || disc.reason.contains("Disconnected") {
+                    return Err(anyhow::anyhow!("ADMIN_DISCONNECT: {}", disc.reason));
+                }
                 return Err(anyhow::anyhow!("Disconnect: {}", disc.reason));
             }