fix: heartbeat button shows proper errors instead of failing silently

- CSRF check returns JSON instead of die() with raw text
- JS parses non-JSON responses gracefully and shows server error
- Visual feedback (check/cross icon) on success/failure
- 3-second icon revert after result

Claude-Session: https://claude.ai/code/session_01Jo2JpjCwfHAh2HHRSjczKq

source/packages/com_mokosuiteclient/admin/src/Controller/DisplayController.php

@@ -86,7 +86,12 @@ class DisplayController extends BaseController
 
 	public function sendHeartbeat()
 	{
-		Session::checkToken() or die(Text::_('JINVALID_TOKEN'));
+		if (!Session::checkToken())
+		{
+			$this->jsonResponse(['success' => false, 'message' => 'Session expired — please reload the page.']);
+
+			return;
+		}
 
 		try
 		{