Clear the markdown live preview after posting a comment

Setting the textarea's .value programmatically does not fire an 'input' event,
so updatePreview() never ran and the preview kept showing the just-posted
comment's rendered markdown underneath an empty composer.

(This change was already present in the working tree at the start of the
session; committing it on its own rather than folding it into an unrelated fix.)

assets/js/ticket.js

@@ -284,7 +284,15 @@ function addComment() {
             // Clear the comment box
             const nc = document.getElementById('newComment');
             if (nc) nc.value = '';
-            
+
+            // Clear the live preview — clearing the textarea programmatically
+            // does not fire 'input', so updatePreview() never runs
+            const previewDiv = document.getElementById('markdownPreview');
+            if (previewDiv) {
+                previewDiv.innerHTML = '';
+                previewDiv.classList.add('is-hidden');
+            }
+
             // Format the comment text for display
             let displayText;
             if (isMarkdownEnabled) {