fix: guard against null msgInput before addEventListener in chat page

If msgInput element is somehow null, addEventListener throws and prevents
loadConvs() from being called — wrapping in null check makes it safe.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

admin/DashboardController.php

@@ -2116,10 +2116,12 @@ function formatDate(d) {
 
 // ─── Auto-resize textarea ────────────────────────────────────────────────────
 const msgInput = document.getElementById('msgInput');
+if (msgInput) {
   msgInput.addEventListener('input', function() {
     this.style.height = 'auto';
     this.style.height = Math.min(this.scrollHeight, 120) + 'px';
   });
+}
 
 loadConvs();
 setInterval(loadConvs, 15000);